Implement quote! and other proc_macro API.

Author: jseyfried

src/Cargo.lock

@@ -0,0 +1,7 @@
+# `proc_macro`
+
+The tracking issue for this feature is: [#38356]
+
+[#38356]: https://github.com/rust-lang/rust/issues/38356
+
+------------------------

src/doc/unstable-book/src/library-features/proc-macro.md

@@ -9,3 +9,4 @@ crate-type = ["dylib"]
 
 [dependencies]
 syntax = { path = "../libsyntax" }
+syntax_pos = { path = "../libsyntax_pos" }