py: Do adjacent str/bytes literal concatenation in lexer, not compiler.
It's much more efficient in RAM and code size to do implicit literal string concatenation in the lexer, as opposed to the compiler. RAM usage is reduced because the concatenation can be done right away in the tokeniser by just accumulating the string/bytes literals into the lexer's vstr. Prior to this patch adjacent strings/bytes would create a parse tree (one node per string/bytes) and then in the compiler a whole new chunk of memory was allocated to store the concatenated string, which used more than double the memory compared to just accumulating in the lexer. This patch also significantly reduces code size: bare-arm: -204 minimal: -204 unix x64: -328 stmhal: -208 esp8266: -284 cc3200: -224