This commit adds syntax extension forms matching the types for procedural macros 2.0 (RFC #1566), these still require the usual syntax extension boiler plate, but this is a first step towards proper implementation and should be useful for macros 1.1 stuff too.
Supports both attribute-like and function-like macros.
Kicking off libproc_macro
This PR introduces `libproc_macro`, which is currently quite bare-bones (just a few macro construction tools and an initial `quote!` macro).
This PR also introduces a few test cases for it, and an additional `shim` file (at `src/libsyntax/ext/proc_macro_shim.rs` to allow a facsimile usage of Macros 2.0 *today*!
