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/isa/x86/abi64.cton

@@ -21,7 +21,7 @@ ebb0:
 
 function %pass_stack_int64(i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64 vmctx) spiderwasm {
     sig0 = (i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64, i64 vmctx) spiderwasm
-    fn0 = sig0 u0:0
+    fn0 = u0:0 sig0
 
 ebb0(v0: i64, v1: i64, v2: i64, v3: i64, v4: i64, v5: i64, v6: i64, v7: i64, v8: i64, v9: i64, v10: i64, v11: i64, v12: i64, v13: i64, v14: i64, v15: i64, v16: i64, v17: i64, v18: i64, v19: i64, v20: i64):
     call fn0(v0, v1, v2, v3, v4, v5, v6, v7, v8, v9, v10, v11, v12, v13, v14, v15, v16, v17, v18, v19, v20)