Remove unused code to old Visual Studio in `tasks/toolchains/visualcpp.rake`

Remove duplicates header files in `src/pool.c`

These are included in `mruby.h`.

As a background, if `enable_cxx_abi` is specified, the macro that
defines the maximum value in `stdint.h` is undefined depending on the
environment.

- Confirmed with gcc on FreeBSD 12.0 and mingw32-gcc available on
  FreeBSD.
- `INTPTR_MAX`, `INT64_MAX` and `UINT64_MAX` are defined by `cstdint`
  added in C++11. But `toolchains :gcc` adds `-std=c++03`, so the macros
  are not defined.
- To have these defined in `C++03`, `__STDC_CONSTANT_MACROS` must be
  defined in advance. This is already done by `mruby.h`.

Add tests for #4746

Fix opcode semantics comment miss.

For example, local variables in the following def:
```ruby
def foo(a = (not_set = true), &block)
  ...
end
```
should be `a, block, not_set`, but were `a, not_set, block`.

Avoid `symhash()` call for inline symbol in `sym_intern()`

Escape the AST string

Dump more node types in `mrb_parser_dump`

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1 2]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001       node type: 85 (0x55)
  00001       NODE_STR "2" len 1
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1 2]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001       NODE_LITERAL_DELIM
  00001       NODE_STR "2" len 1
  (snip)
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 87 (0x57)
  00001     node type: 87 (0x57)
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_SYMBOLS:
  00001       NODE_STR "1" len 1
  00001     NODE_SYMBOLS:
  00001       NODE_STR "" len 0
  00001       NODE_BEGIN:
  00001         NODE_INT 2 base 10
  00001       NODE_STR "" len 0
  (snip)
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1]; %W[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 86 (0x56)
  00001     node type: 86 (0x56)
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1]; %W[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001     NODE_WORDS:
  00001       NODE_STR "" len 0
  00001       NODE_BEGIN:
  00001         NODE_INT 2 base 10
  00001       NODE_STR "" len 0
  (snip)
  ```

Remove a trailing space in `MRUBY_DESCRIPTION`

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e ':"#{1}"; ["#{2}": 0]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 83 (0x53)
  00001     NODE_ARRAY:
  00001       NODE_KW_HASH:
  00001         key:
  00001           node type: 83 (0x53)
  00001         value:
  00001           NODE_INT 0 base 10
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e ':"#{1}"; ["#{2}": 0]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_DSYM:
  00001       NODE_DSTR:
  00001         NODE_STR "" len 0
  00001         NODE_BEGIN:
  00001           NODE_INT 1 base 10
  00001         NODE_STR "" len 0
  00001     NODE_ARRAY:
  00001       NODE_KW_HASH:
  00001         key:
  00001           NODE_DSYM:
  00001             NODE_DSTR:
  00001               NODE_STR "" len 0
  00001               NODE_BEGIN:
  00001                 NODE_INT 2 base 10
  00001               NODE_STR "" len 0
  00001         value:
  00001           NODE_INT 0 base 10
  (snip)
  ```

Remove unnecessary type `mrb_hash_value`

The type `mrb_hash_value` is no longer used by the segmented list
implementation (ref e8dcfe17 and e65d4264).

Print missing `:` before newline in `mrb_parser_dump`

Allow rethrowing `MRB_TT_BREAK`

Restore GC arena frequently

Remove unnecessary assignments

Add an annotation about the return value

The return value from `mrb_delete_key` needs to be protected from GC in
some cases.

Remove unused node type in `codegen()`