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/cretonne/src/binemit/relaxation.rs

@@ -148,7 +148,7 @@ fn relax_branch(
     // Pick the first encoding that can handle the branch range.
     let dfg = &cur.func.dfg;
     let ctrl_type = dfg.ctrl_typevar(inst);
-    if let Some(enc) = isa.legal_encodings(dfg, &dfg[inst], ctrl_type).find(
+    if let Some(enc) = isa.legal_encodings(cur.func, &dfg[inst], ctrl_type).find(
         |&enc| {
             let range = encinfo.branch_range(enc).expect("Branch with no range");
             if !range.contains(offset, dest_offset) {