0.4.0

Track upstream proc_macro changes.

• Rename Term to Ident
• Rename Punct to Op
• Remove Term::as_str
• Rename Op::op to Punct::as_char
• Term::new no longer accepts lifetimes or raw idents
• Lifetimes are lexed as a Joint ' character followed by an Ident
• Ident::new_raw is a new procmacro2_semver_exempt API for creating raw
  identifiers.
• {Eq,PartialEq,Ord,PartialOrd,Hash} for Ident
• Remove Copy for Term
• Remove Copy for Op
• Add Extend<TokenTree> for TokenStream