Rename FunctionRetTy -> ReturnType

The FunctionRetTy naming was copied from libsyntax. The new name uses 2 fewer
non-standard abbreviations (where standard means used in Rust source code, like
`impl` or `mod`).
diff --git a/src/lib.rs b/src/lib.rs
index 19e5998..5e86bc4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -84,7 +84,7 @@
 
 mod ty;
 pub use ty::{Abi, AbiKind, AngleBracketedParameterData, BareFnArg, BareFnArgName, BareFnTy,
-             FunctionRetTy, MutTy, Mutability, ParenthesizedParameterData, Path,
+             ReturnType, MutTy, Mutability, ParenthesizedParameterData, Path,
              PathParameters, PathSegment, PolyTraitRef, QSelf, Ty, TypeBinding, Unsafety,
              TySlice, TyArray, TyPtr, TyRptr, TyBareFn, TyNever, TyTup, TyPath,
              TyTraitObject, TyImplTrait, TyParen, TyInfer, TyGroup};