This was for pre Ruby 2.4 support which we dropped a while ago.

While it's a very nice API, if the buffer is too big for the
stack (> 1024B) the buffer object will be mark like a stack region
which is slow and overkill for a char buffer.

test/json/json_parser_test.rb:141: warning: Integer out of Float range
lib/json/common.rb:353: warning: Float 12345678901234567890... out of range

Ref: #939

Instead of using %{} which works like double-quoted string and allows
escape sequences and in which "other escaped characters (a backslash
followed by a character) are interpreted as the character", change the
examples for `allow_invalid_escape` to use single-quotes strings, in
which "any other character following a backslash [except ' and \] is
interpreted as is: a backslash and the character itself."

This makes it clearer the JSON document only includes a single
backslash.

Ref: #946

unsigned long is only 32b on some platforms.

Ref: fff25c9

`StringValuePtr` use `volatile` so the compiler is less likely
to re-use the register.

But regardless, we should GC_GUARD `str` as we no longer reference
it after `GETMEM`.