Change the URL of `mruby.org` [ci skip]

- `http` -> `https`
- Remove `www.`

Support `--` (end of options) to `mruby` command

#### Before this patch:

  ```
  $ bin/mruby -e 'p ARGV' -- -x
  bin/mruby: invalid option -- (-h will show valid options)
  ```

#### After this patch:

  ```
  $ bin/mruby -e 'p ARGV' -- -x
  ["-x"]
  ```

Add assertion to `RVALUE` size

Rename `BITSIZE` to `BIT` and `BIT` to `BIT_POS` for consistency

Fix `ARGV` value in `mruby` command (regression by #4827)

#### Before this patch:

  ```
  $ bin/mruby -e 'p ARGV' a b
  ["bin/mruby", "-e", "p ARGV", "a", "b"]
  ```

#### After this patch:

  ```
  $ bin/mruby -e 'p ARGV' a b
  ["a", "b"]
  ```

Support short options concatenation to `mruby` command

Remove unused methods of `MRubyIOTestUtil`

Use proper `PEEK` macro for `OP_EPUSH` in `patch_irep`; fix #4833

The bit width terminology is unified to `BIT` according to `MRB_INT_BIT`
and `CHAR_BIT`. Also the bit position terminology is unified to `BIT_POS`.

Allow `true`/`false` argument to `Kernel#exit`

Introduce `mrb_ssize` type for buffer size on memory; ref #4483

Previously, `mrb_int` was used as the type that represents the buffer size
on memory, but the sizes of `RString` and `RArray` exceed 6 words when
`MRB_INT64` is enabled on 32-bit CPU.

I don't think it is necessary to be able to represent the buffer size on
memory that exceeds the virtual address space. Therefore, for this purpose,
introduce `mrb_ssize` which doesn't exceed the sizes of `mrb_int` and
pointer.

I think all `mrb_int` used for this purpose should be changed to
`mrb_ssize`, but currently only the members of the structures (`RString`,
`mrb_shared_string`, `RArray` and `mrb_shared_array`) are changed.

Always enable the rational and complex literals

Release `mruby 2.1.0`.

I think they can always be enabled because the regular expression literal is
always enabled.

Refactor `mrb_string_value_cstr`

- Keep `MRB_STR_ASCII` flag.
- Avoid a string object creation.

Fix argument specs to `Kernel`

#### Before this patch:

  ```
  $ bin/mruby -ce 1
  bin/mruby: Cannot open program file: 1
  ```

#### After this patch:

  ```
  $ bin/mruby -ce 1
  Syntax OK
  ```

Revert "Implement Ruby2.7's frozen strings from `Symbol#to_s`"

This feature was reverted from Ruby 2.7.

Fix here document with EOF

#### Before this patch:

  ```
  $ bin/mruby -e 'p <<EOS
1
EOS'
  -e:4:0: can't find heredoc delimiter "EOS" anywhere before EOF
  -e:4:0: syntax error, unexpected $end, expecting tHEREDOC_END or tHD_STRING_PART or tHD_STRING_MID
  ```

#### After this patch (same as Ruby):

  ```
  $ bin/mruby -e 'p <<EOS
1
EOS'
  "1\n"
  ```

Use `mrb_intern_lit` if possible in `parse.y`

Use `intern` instead of `intern_cstr` if possible in `parse.y`

Fix argument specs to `Integer`