Rename Mac -> Macro

This is part of a move to standardize on abbreviating only those types in the
AST that are abbreviated in Rust source code, like `impl` or `fn`. The grammar
of Rust source code never abbreviates `macro` to `mac`, so this commit reflects
that.

Field names are being kept as `mac` because `macro` is a reserved word.
diff --git a/src/ty.rs b/src/ty.rs
index d599283..2f671a6 100644
--- a/src/ty.rs
+++ b/src/ty.rs
@@ -77,7 +77,7 @@
             pub underscore_token: tokens::Underscore
         }),
         /// A macro in the type position.
-        pub Mac(Mac),
+        pub Macro(Macro),
     }
 }
 
@@ -362,7 +362,7 @@
         syn!(TyParen) => { Ty::Paren }
         |
         // must be before path
-        syn!(Mac) => { Ty::Mac }
+        syn!(Macro) => { Ty::Macro }
         |
         // must be before ty_poly_trait_ref
         call!(ty_path, allow_plus)