Parse multiple decls/stmts inside of injections

[asutton]

Support this:

constexpr {
  -> {
    void f() { }
    void g() { }
  }
}

I believe the parser will currently lex all contained declarations and statements, but we need to ensure that they are processed as multiple declarations or statements.

[asutton]

This is related to #25. Fixing that will fix this also.