Binary function names (#91)

* Function names should start with %

* Create FunctionName from string

* Implement displaying of FunctionName as %nnnn with fallback to #xxxx

* Run rustfmt and fix FunctionName::with_string in parser

* Implement FunctionName::new as a generic function

* Binary function names should start with #

* Implement NameRepr for function name

* Fix examples in docs to reflect that function names start with %

* Rebase and fix filecheck tests

cranelift/filetests/parser/keywords.cton

@@ -1,5 +1,5 @@
 test cat
 
 ; 'function' is not a keyword, and can be used as the name of a function too.
-function function() {}
-; check: function function()
+function %function() {}
+; check: function %function()