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/isa/riscv/encoding.cton

@@ -1,7 +1,7 @@
 test legalizer
 isa riscv supports_m=1
 
-function int32(i32, i32) {
+function %int32(i32, i32) {
 ebb0(v1: i32, v2: i32):
     v10 = iadd v1, v2
     ; check: [R#0c]