Implemented tokenise_buffer

Note that this is basically the same as the previous version,
excluding the fact that it uses C++ idioms more and does a bit better
in error checking.

asm/lexer.hpp

@@ -91,6 +91,6 @@ enum lerr_t
 };
 const char *lerr_as_cstr(lerr_t);
 
-lerr_t tokenise_string(std::string_view, std::vector<token_t> &);
+lerr_t tokenise_buffer(std::string_view, std::vector<token_t> &);
 
 #endif