py: Optimise lexer by exposing lexer type.
mp_lexer_t type is exposed, mp_token_t type is removed, and simple lexer functions (like checking current token kind) are now inlined. This saves 784 bytes ROM on 32-bit unix, 348 bytes on stmhal, and 460 bytes on bare-arm. It also saves a tiny bit of RAM since mp_lexer_t is a bit smaller. Also will run a bit more efficiently.