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/parse-encoding.cton

@@ -27,9 +27,9 @@ function %parse_encoding(i32 [%x5]) -> i32 [%x10] {
     ; check: sig5 = () -> f32 [0] system_v
 
     ; function + signature
-    fn0 = function %bar(i32 [%x10]) -> b1 [%x10] system_v
+    fn0 = %bar(i32 [%x10]) -> b1 [%x10] system_v
     ; check: sig6 = (i32 [%x10]) -> b1 [%x10] system_v
-    ; nextln: fn0 = sig6 %bar
+    ; nextln: fn0 = %bar sig6
 
 ebb0(v0: i32):
     return v0