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/isa/riscv/abi-e.cton

@@ -4,7 +4,7 @@ isa riscv enable_e
 
 ; regex: V=v\d+
 
-function f() {
+function %f() {
     ; Spilling into the stack args after %x15 since %16 and up are not
     ; available in RV32E.
     sig0 = signature(i64, i64, i64, i64) -> i64