reflect: add support for linked lists

Linked lists are usually implemented as follows:

    type linkedList struct {
        next *linkedList
        data int // whatever
    }

This caused a stack overflow while writing out the reflect run-time type
information. This has now been fixed by splitting the allocation of a
named type number from setting the underlying type in the sidetable.

testdata/reflect.go

@@ -22,6 +22,10 @@ type (
 		buf  []byte
 		Buf  []byte
 	}
+	linkedList struct {
+		next *linkedList `description:"chain"`
+		foo  int
+	}
 )
 
 func main() {
@@ -103,6 +107,9 @@ func main() {
 			c int8
 		}{42, 321, 123},
 		mystruct{5, point{-5, 3}, struct{}{}, []byte{'G', 'o'}, []byte{'X'}},
+		&linkedList{
+			foo: 42,
+		},
 	} {
 		showValue(reflect.ValueOf(v), "")
 	}

testdata/reflect.txt

@@ -293,6 +293,22 @@ reflect type: struct
     indexing: 0
     reflect type: uint8 settable=true
       uint: 88
+reflect type: ptr
+  pointer: true struct
+  nil: false
+  reflect type: struct settable=true
+    struct: 2
+    field: 0 next
+    tag: description:"chain"
+    embedded: false
+    reflect type: ptr
+      pointer: false struct
+      nil: true
+    field: 1 foo
+    tag: 
+    embedded: false
+    reflect type: int
+      int: 42
 
 sizes:
 int8 1 8