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/reload-208.cton

@@ -16,8 +16,8 @@ function %pr208(i64 vmctx [%rdi]) system_v {
     heap0 = static gv0, min 0, bound 0x5000, guard 0x0040_0000
     sig0 = (i64 vmctx [%rdi]) -> i32 [%rax] system_v
     sig1 = (i64 vmctx [%rdi], i32 [%rsi]) system_v
-    fn0 = sig0 u0:1
-    fn1 = sig1 u0:3
+    fn0 = u0:1 sig0
+    fn1 = u0:3 sig1
 
 ebb0(v0: i64):
     v1 = iconst.i32 0