Implement cap, len, and make, as well as the general framework

for built-in functions and type conversions.  Extract out
common operations on expression nodes for converting them to
ints and implicitly dereferencing arrays.

R=rsc
APPROVED=rsc
DELTA=442  (365 added, 50 deleted, 27 changed)
OCL=34064
CL=34064

usr/austin/eval/abort.go

@@ -67,5 +67,21 @@ type KeyNotFound struct {
 }
 
 func (e KeyNotFound) String() string {
-	return fmt.Sprintf("key %s not found in map", e.Key);
+	return fmt.Sprintf("key '%v' not found in map", e.Key);
+}
+
+type NegativeLength struct {
+	Len int64;
+}
+
+func (e NegativeLength) String() string {
+	return fmt.Sprintf("negative length: %d", e.Len);
+}
+
+type NegativeCapacity struct {
+	Len int64;
+}
+
+func (e NegativeCapacity) String() string {
+	return fmt.Sprintf("negative capacity: %d", e.Len);
 }