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/regalloc/basic.cton

@@ -53,7 +53,7 @@ ebb1(v10: i32):
 
 ; Pass an EBB argument as a function argument.
 function %callebb(i32, i32) -> i32 {
-    fn0 = function %foo(i32) -> i32
+    fn0 = %foo(i32) -> i32
 
 ebb0(v1: i32, v2: i32):
     brnz v1, ebb1(v1)
@@ -66,7 +66,7 @@ ebb1(v10: i32):
 
 ; Pass an EBB argument as a jump argument.
 function %jumpebb(i32, i32) -> i32 {
-    fn0 = function %foo(i32) -> i32
+    fn0 = %foo(i32) -> i32
 
 ebb0(v1: i32, v2: i32):
     brnz v1, ebb1(v1, v2)