Add span to lex errors

[dtolnay]

Closes #178.

This PR adds impl LexError { pub fn span(&self) -> Span }. While libproc_macro's LexError does not have such a method, having this is far more important for proc-macro2 than for libproc_macro, because proc macro input is already lexed by rustc and receives good rustc-generated diagnostics on lex errors, while main.rs / build.rs-based parser do not get that.

Before

  error: lex error
    ┌─ src/main.rs:1:1
    │
  1 │ //! this file contains a lex error... where?
    │ ^ lex error

After

  error: lex error
     ┌─ src/main.rs:46:12
     │
  46 │ fn main() {)
     │            ^ lex error