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/riscv/binary32.cton

@@ -4,7 +4,7 @@ isa riscv
 
 function %RV32I(i32 link [%x1]) -> i32 link [%x1] {
     sig0 = ()
-    fn0 = function %foo()
+    fn0 = %foo()
 
 ebb0(v9999: i32):
     [-,%x10]            v1 = iconst.i32 1