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

filetests/domtree/basic.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function test(i32) {
+function %test(i32) {
     ebb0(v0: i32):
         jump ebb1     ; dominates: ebb1
     ebb1: