https://bugs.ruby-lang.org/issues/21785#note-10
> It is too late to introduce it in Ruby 4.0, let's aim for 4.1.

This reverts commits:

* d0b72429a93e54f1f956b4aedfc25c57dc7001aa
  Add support for signed and unsigned LEB128 to pack/unpack.

* 68a900e30b4ca1537d7975c3a619fd94fca7b084
  add news for pack / unpack directives

This commit adds a new pack format command `R` and `r` for unsigned and
signed LEB128 encoding.  The "r" mnemonic is because this is a
"vaRiable" length encoding scheme.

LEB128 is used in various formats including DWARF, WebAssembly, MQTT,
and Protobuf.

[Feature #21785]

Use an enum for the method arg instead of needing to add an id
that doesn't map to an actual method name.

$ ruby --dump=insns -e 'b = "x"; [v].pack("E*", buffer: b)'

before:

```
== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,34)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] b@0
0000 putchilledstring                       "x"                       (   1)[Li]
0002 setlocal_WC_0                          b@0
0004 putself
0005 opt_send_without_block                 <calldata!mid:v, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0007 newarray                               1
0009 putchilledstring                       "E*"
0011 getlocal_WC_0                          b@0
0013 opt_send_without_block                 <calldata!mid:pack, argc:2, kw:[#<Symbol:0x000000000023110c>], KWARG>
0015 leave
```

after:

```
== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,34)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] b@0
0000 putchilledstring                       "x"                       (   1)[Li]
0002 setlocal_WC_0                          b@0
0004 putself
0005 opt_send_without_block                 <calldata!mid:v, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0007 putchilledstring                       "E*"
0009 getlocal                               b@0, 0
0012 opt_newarray_send                      3, 5
0015 leave
```

Instructions for this code:

```ruby
  # frozen_string_literal: true

[a].pack("C")
```

Before this commit:

```
== disasm: #<ISeq:<main>@test.rb:1 (1,0)-(3,13)>
0000 putself                                                          (   3)[Li]
0001 opt_send_without_block                 <calldata!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 newarray                               1
0005 putobject                              "C"
0007 opt_send_without_block                 <calldata!mid:pack, argc:1, ARGS_SIMPLE>
0009 leave
```

After this commit:

```
== disasm: #<ISeq:<main>@test.rb:1 (1,0)-(3,13)>
0000 putself                                                          (   3)[Li]
0001 opt_send_without_block                 <calldata!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 putobject                              "C"
0005 opt_newarray_send                      2, :pack
0008 leave
```

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

`RbConfig::SIZEOF` has been added later than the time this check was
committed first.

It is required in the always defined class level.

* [Bug #19150]

[Bug #19150]