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
```