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/parser/rewrite.cton

@@ -9,13 +9,13 @@
 test cat
 
 ; Check that defining numbers are rewritten.
-function defs() {
+function %defs() {
 ebb100(v20: i32):
     v1000 = iconst.i32x8 5
     v9200 = f64const 0x4.0p0
     trap
 }
-; sameln: function defs() {
+; sameln: function %defs() {
 ; nextln: $ebb100($v20: i32):
 ; nextln:     $v1000 = iconst.i32x8 5
 ; nextln:     $v9200 = f64const 0x1.0000000000000p2
@@ -23,13 +23,13 @@ ebb100(v20: i32):
 ; nextln: }
 
 ; Using values.
-function use_value() {
+function %use_value() {
 ebb100(v20: i32):
     v1000 = iadd_imm v20, 5
     v200 = iadd v20, v1000
     jump ebb100(v1000)
 }
-; sameln: function use_value() {
+; sameln: function %use_value() {
 ; nextln: ebb0($v20: i32):
 ; nextln:     $v1000 = iadd_imm $v20, 5
 ; nextln:     $v200 = iadd $v20, $v1000