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:

filetests/domtree/loops.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function test(i32) {
+function %test(i32) {
     ebb0(v0: i32):
         brz v0, ebb1  ; dominates: ebb1 ebb3 ebb4 ebb5
         jump ebb2     ; dominates: ebb2

filetests/domtree/loops2.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function test(i32) {
+function %test(i32) {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb6
         brnz v0, ebb2   ; dominates: ebb2 ebb9

filetests/domtree/tall-tree.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function test(i32) {
+function %test(i32) {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1
         brnz v0, ebb2   ; dominates: ebb2 ebb5

filetests/domtree/wide-tree.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function test(i32) {
+function %test(i32) {
     ebb0(v0: i32):
         brz v0, ebb13   ; dominates: ebb13
         jump ebb1       ; dominates: ebb1