Move jointness censoring to proc_macro
Proc-macro API currently exposes jointness in `Punct` tokens. That is,
`+` in `+one` is **non** joint.
Our lexer produces jointness info for all tokens, so we need to censor
it *somewhere*
Previously we did this in a lexer, but it makes more sense to do this
in a proc-macro server.
r? @petrochenkov