Reimplement public interface for stability

More information to come later about this, but this is a result of the
work week discussions we've had about stabilizing procedural macros

Author: alexcrichton

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "proc-macro2"
-version = "0.2.3" # remember to update html_root_url
+version = "0.3.0" # remember to update html_root_url
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT/Apache-2.0"
 readme = "README.md"