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/binary32.cton

@@ -10,7 +10,7 @@ isa x86 haswell
 
 function %I32() {
     sig0 = ()
-    fn0 = function %foo()
+    fn0 = %foo()
 
     gv0 = globalsym %some_gv