embed_string.h

[eggyknap]

I've just committed a file called embed_string.h which aims to declare
a set of Parrot_String functions we'll want. I've no idea if I've done
it right (first guess: I haven't), but it compiles on my box.

Perhaps I'm misunderstanding how Parrot_compile_string() is supposed
to work, but the Parrot_String pointer I feed it in plparrot.c still
comes back containing a NULL Parrot_String (as per STRING_is_null())
when I try to compile PIR full of syntax errors.