Create built-in instruction macros `%hex("...")` and `%string("...")`

[SamWilsn]

%hex("...")

An instruction macro that inserts the argument literally into the assembled output:

jumpdest
%hex("7FAB")
invalid

Would become:

5b7fabfe

We already have %include_hex, but this would be useful for smaller snippets.

%string("...")

An instruction macro that inserts the argument literally into the assembled output, encoded as UTF-8:

jumpdest
%string("hello world")
invalid

Would become:

5b68656c6c6f20776f726c64fe

This macro is useful for revert reasons.