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.

lib/filetests/src/test_binemit.rs

@@ -133,7 +133,7 @@ impl SubTest for TestBinEmit {
                     // constraints.
                     if let Some(enc) = {
                         let mut legal_encodings = isa.legal_encodings(
-                            &func.dfg,
+                            &func,
                             &func.dfg[inst],
                             func.dfg.ctrl_typevar(inst),
                         ).filter(|e| {
@@ -251,7 +251,7 @@ impl SubTest for TestBinEmit {
 
                         // Do any encodings exist?
                         let encodings = isa.legal_encodings(
-                            &func.dfg,
+                            &func,
                             &func.dfg[inst],
                             func.dfg.ctrl_typevar(inst),
                         ).map(|e| encinfo.display(e))