1213: Make lexer produce only single character puncts r=matklad a=edwin0cheng
As discussed in Zulip, this PR change `lexer` to produce only single char punct.
* Remove producing `DOTDOTDOT, DOTDOTEQ, DOTDOT, COLONCOLON, EQEQ, FAT_ARROW, NEQ, THIN_ARROW` in lexer.
* Add required code in parser to make sure everythings works fine.
* Change some tests (Mainly because the `ast::token_tree` is different)
Note: i think the use of `COLON` in rust is too overloaded :)
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
6618d1edc3