So that the function behave consistently with `mrb_load_proc()`.

- String#__lines
- Array#__ary_eq
- Array#__ary_cmp
- Hash#__delete
- Kernel#__case_eqq
- Integer#__coerce_step_counter

Mrbgem gem_init.c: add headers for irep loading when cdump is disabled, but presym is enabled

Do not include `stdint.h` before `mruby.h`

When `disable_cdump` is declared on a Mrbgem spec, the procedure for
loading MRuby code from the gem's `mrblib` directory is slightly
different; instead of loading the mrblib code as a Proc built from the
compiled irep, the compiled irep is loaded directly. The header files
`mruby.h` and `mruby/proc.h` are needed only when the irep is loaded
directly. They are currently included only when presym is disabled, they
should be included whenever either presym is disabled *or* when
`disable_cdump` is called. The `cdump?` predicate method happens to
return true in either case (presym disabled *or* cdump disabled), so
this change should be safe.

A change in `load.c` is left uncommitted.

That means it's a method not to be included in the backtrace, for
example `raise`.

It uses BER number compression of delta of instruction positions and line
numbers. BER compression is a variable length number representation.

* `mrb_debug_line_ary`: array of line numbers represented in `uint16_t`.
  `[lineno, lineno, ...]`

* `mrb_debug_line_flat_map`: array of `mrb_irep_debug_info_line`, which
  is `struct {uint32_t pos; uint16_t lineno}`, for each line.

* `mrb_debug_line_packed_map` [new]: sequence of BER compressed 2
  numbers, `pos_delta, lineno_delta`. Deltas are differences from
  previous values (starting `0`). `line_entry_counts` represents total
  length of a packed map string for this type.

Add information on yamllint as comments

Style the CONTRIBUTING guide

Merge branch 'mrb_debug_strdup-and-strndup' of https://github.com/cremno/mruby into cremno-mrb_debug_strdup-and-strndup

To be compatible with CRuby.

- `Lit` -> `Pool`
- `SEQ` -> `Irep`

The code generator no longer need to emit `OP_LOADSYM` after `OP_DEF`.
`doc/opcode.md` is also updated.

This reverts commit fd10c723.

I thought it was OK to restrict index value within 1 byte, but in some
cases index value could be 16 bits (2 bytes). I had several ideas to
address the issue, but reverting `fd10c723` is the easiest way. The
biggest reason is `mruby/c` still supports `OP_EXT[123]`, so that they
don't need any additional work.

Drop unnecessary upper procs linked from class/module/def syntax

Avoid warnings with `ruby -cw`

```console
% for rb in `git ls-files '*/mrblib/*.rb' 'mrblib'`; do ruby30 -cw $rb > /dev/null; done
mrbgems/mruby-array-ext/mrblib/array.rb:389: warning: assigned but unused variable - ary
mrbgems/mruby-array-ext/mrblib/array.rb:663: warning: assigned but unused variable - len
mrbgems/mruby-hash-ext/mrblib/hash.rb:119: warning: possibly useless use of a variable in void context
mrbgems/mruby-hash-ext/mrblib/hash.rb:259: warning: assigned but unused variable - keys
mrbgems/mruby-io/mrblib/io.rb:229: warning: literal in condition
mrbgems/mruby-io/mrblib/io.rb:280: warning: literal in condition
mrbgems/mruby-string-ext/mrblib/string.rb:347: warning: assigned but unused variable - len
mrbgems/mruby-toplevel-ext/mrblib/toplevel.rb:2: warning: parentheses after method name is interpreted as an argument list, not a decomposed argument
```