Should `5.f` parse as a float literal, or expr_field(int literal, "f")?

We can make the lexer not consider a dot part of a number when followed immediately by a letter. This makes `5.f`, `10.f64`, and similar invalid, but the dot is already redundant there anyway.

This allows us to do `10.some_method_name()`. That'd be a relatively rare thing to write, but allowing it is nice, and it removes some kludgy code from the pretty-printer.
