get rid of C-strings for STRING_TYPE constants in PIRC

[plobsing]

Originally reported in parrot TT #172 (http://trac.parrot.org/parrot/ticket/172)

strings are stored in PIRC as c-strings, just arrays of bytes. I should get rid of my stubbornness, and just convert c-strings immediately to STRINGs in the lexer.

(It might even simplify some code)

This is necessary as PIRC needs some functionality that only deals with (or returns) STRINGs, not char *.