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/docs/callex.cton

@@ -1,7 +1,7 @@
 test verifier
 
 function %gcd(i32 uext, i32 uext) -> i32 uext system_v {
-    fn1 = function %divmod(i32 uext, i32 uext) -> i32 uext, i32 uext
+    fn1 = %divmod(i32 uext, i32 uext) -> i32 uext, i32 uext
 
 ebb1(v1: i32, v2: i32):
     brz v2, ebb2