Add FunctionName, Signature to repr::Function.

Simplify the uses in parser.rs to avoid too many module qualifiers.

src/libcretonne/types.rs

@@ -194,6 +194,12 @@ impl Display for Type {
 //
 // ====--------------------------------------------------------------------------------------====//
 
+/// The name of a function can be any UTF-8 string.
+///
+/// Function names are mostly a testing and debugging tool. In partucular, `.cton` files use
+/// function names to identify functions.
+pub type FunctionName = String;
+
 /// Function argument extension options.
 ///
 /// On some architectures, small integer function arguments are extended to the width of a