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.cton

@@ -5,7 +5,7 @@ isa riscv enable_e
 
 ; Check that we can handle a function return value that got spilled.
 function %spill_return() -> i32 {
-    fn0 = function %foo() -> i32 system_v
+    fn0 = %foo() -> i32 system_v
 
 ebb0:
     v0 = call fn0()