Place optable in ROM

refactor: add the spell checker Action to the Lint Action file

Order jobs in workflow.

chore: fix missing HTTPS on links

chore: fix spelling

Normally a single spell checker can't find all the mistakes or check all types of code.

These mistakes were found by another spell checker inside my editor with a more manual sift / find.

feat(CI): check for trailing whitespace

Run on pull request only.
Use a shell script to check for trailing whitespace in all files.
Fail if trailing whitespace is found.

refactor: only use certified GitHub Actions to increase security

Official -> "The individual jobs in a workflow can interact with (and compromise) other jobs. For example, a job querying the environment variables used by a later job, writing files to a shared directory that a later job processes, or even more directly by interacting with the Docker socket and inspecting other running containers and executing commands in them.

This means that a compromise of a single action within a workflow can be very significant, as that compromised action would have access to all secrets configured on your repository, and can use the GITHUB_TOKEN to write to the repository. Consequently, there is significant risk in sourcing actions from third-party repositories on GitHub. "

https://docs.github.com/en/actions/learn-github-actions/security-hardening-for-github-actions#using-third-party-actions

Update compile prerequisites in `doc/guides/compile.md` [ci skip]

Check if `esclen` is smaller than `len` (original string length).

Bison and gperf are optional.

- Remove Ubuntu-16.04
- Add Ubuntu-20.04 {gcc,clang}

Fix build `mruby-catch` gem without presym

Add `UncaughtThrowError#{tag,value}` for Ruby compatibility

Remove "skip ci" hack to GitHub Actions [skip travis][skip appveyor]

GitHub Actions now supports "skip ci" natively:
https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

It used to return `mrb_ssize` but its size may differ from `mrb_int`,
e.g. `MRB_INT64` on `MRB_32BIT` architecture.

Merge pull request #5334 from shuujii/avoid-possibility-of-reading-uninitialized-areas-in-h_check_modified

Avoid possibility of reading uninitialized areas in `h_check_modified()`

In `h_check_modified()`, in the case of `MRB_NO_BOXING`, `ht_ea()` or
`ht_ea_capa()` for AR may read uninitialized area. Therefore, do not use
those macros for AR in `MRB_NO_BOXING` (but in the case of `MRB_64BIT`,
`ht_ea_capa()` is the same as `ar_ea_capa()`, so use it).

fix #5332

Binary gems description added, along with a few cosmetic changes.

Binary gems are mrbgems that set `spec.bins` in their `mrbgem.rake`,
and usually their names are prefixed with `mruby-bin-`.

Especially description regarding the build process and `build` directory
structures, along with some typo fixes and cosmetic changes.

Fix heap-buffer-overflow for small `Hash` (HT) in `Hash#rehash`

- Move `#catch` definition to `mruby-catch.c` to avoid tweaking
- Remove `#__preserve_catch_method`
- Implement whole `#throw` method in C

### Example

##### example.rb

```ruby
h = {}
(1..17).each{h[_1] = _1}
(1..16).each{h.delete(_1)}
h.rehash
```

##### ASAN report

```console
$ bin/mruby example.rb
==52587==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000006998 at pc 0x55a29cddf96b bp 0x7fff7b1b1720 sp 0x7fff7b1b1710
READ of size 4 at 0x602000006998 thread T0
    #0 0x55a29cddf96a in ib_it_next /mruby/src/hash.c:639
    #1 0x55a29cde2ca2 in ht_rehash /mruby/src/hash.c:900
    #2 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #3 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #4 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #5 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #6 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #7 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #8 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #9 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #10 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #11 0x55a29cdb4a6d in _start (/mruby/bin/mruby+0x2a3a6d)

0x602000006998 is located 0 bytes to the right of 8-byte region [0x602000006990,0x602000006998)
allocated by thread T0 here:
    #0 0x7f24f01cfffe in __interceptor_realloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dffe)
    #1 0x55a29ceb9440 in mrb_default_allocf /mruby/src/state.c:68
    #2 0x55a29cdba747 in mrb_realloc_simple /mruby/src/gc.c:228
    #3 0x55a29cdba928 in mrb_realloc /mruby/src/gc.c:242
    #4 0x55a29cde12e5 in ht_init /mruby/src/hash.c:749
    #5 0x55a29cde2b8e in ht_rehash /mruby/src/hash.c:897
    #6 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #7 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #8 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #9 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #10 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #11 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #12 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #13 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #14 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
```

Ruby 2.5 is the oldest maintained version.