make clean error workaround

Take commit hash of mruby too

Separate repos directory and build directory

closes #4652

Support lock file for git.

Prioritize embedded string over nofree (or normal) string

Prioritize embedded string in the following functions:

- `str_new_static`
- `str_new`
- `mrb_str_new_capa`
- `mrb_str_pool`

The reasons are as follows:

- Consistency with `mrb_str_byte_subseq` and `str_replace`.
- Memory locality increases and may be slightly faster.
- No conversion cost to embedded string when modifying the string.

Remove deprecated iij stuffs

Fix link of mruby-io

Use mruby-onig-regexp instead in example

Add note about checksum_hash in mrbgem doc

Based on code proposed by @dearblue to avoid the warning:
`dereferencing type-punned pointer will break strict-aliasing rules`.

Use `RBasic` padding for embedded string on 64-bit CPU

On 64-bit CPU, there is padding in `RBasic`, so reorder the fields and use
it as buffer of embedded string. This change allows 4 more bytes to be
embedded on 64-bit CPU.

However, an incompatibility will occur if `RString::as::ary` is accessed
directly because `RString` structure has changed.

Move `mrb_str_pool` to `src/string.c` to use `str_init` family

Allow external definition of MRB_API macro

When building WebAssembly targets with clang (e.g. wasm32-unknown-unknown), all
symbols are hidden by default. To export a symbol, it must be marked with

    __attribute__((visibility(default)))

Rather than detecting this exotic target in mruby, allow MRB_API to be defined
externally.

Suppress warnings for `strncat()`

Simplify get arguments

Generate doxygen docs for mruby

Make symbolic link names unique for test

Rename `mrb_shared_string::len` to `mrb_shared_string::capa`

Also use `str_init_shared` for `orig` in `str_make_shared()`

Prohibit changes to iseq in principle

Because this field is used as capacity of string buffer.

`strncat()` also needs `'\0'`.

- `mrb_str_index_m()` and `mrb_str_rindex()`
  Make `mrb_get_args()` called only once from called twice.
- `mrb_str_byteslice()`
  Replace `goto` with `if ~ else`.