Add a "colocated" flag to symbol references. (#298)

This adds a "colocated" flag to function and symbolic global variables which
indicates that they are defined along with the current function, so they can
use PC-relative addressing.

This also changes the function decl syntax; the name now always precedes the
signature, and the "function" keyword is no longer included.

cranelift/filetests/verifier/type_check.cton

@@ -42,14 +42,14 @@ function %type_mismatch_controlling_variable() {
 }
 
 function %fn_call_too_few_args() {
-    fn2 = function %great_fn(i32, f32)
+    fn2 = %great_fn(i32, f32)
     ebb0:
         call fn2() ; error: mismatched argument count, got 0, expected 2
         return
 }
 
 function %fn_call_too_many_args() {
-    fn5 = function %best_fn()
+    fn5 = %best_fn()
     ebb0:
         v0 = iconst.i64 56
         v1 = f32const 0.0