Rename lexer.c's T_INITIAL to T_START.

This avoids conflict with parser.c's T_INITIAL which we'd like
to move to parser_interal.h. Ideally the lexer would be independent
which would let us change the name back.
Move parser state and mode enums to the header.

This allows us to use the enums in the state structures which
gives us both type checking, and more readable debugger output.
Moved lexer states into parser_internal.h

Renamed lexer states to begin with L_ rather than T_, so that they may
be distinguished from parser states.
Merge pull request #27 from rillian/state-enum

Use enum types for parse state