all: make handling of zero-value composites more consistent

We occasionally need to work with immutable, empty lists, maps, and
messages. Notably, Message.Get on an empty repeated field will return a
"frozen" empty value.

Move handling of these immutable, zero-length composites into Converter,
to unify the behavior of regular and extension fields.

Add a Zero method to Converter, MessageType, and ExtensionType, to
provide a consistent way to get an empty, frozen value of a composite
type. Adding this method to the public {Message,Extension}Type
interfaces does increase our API surface, but lets us (for example)
cleanly represent an empty map as a nil map rather than a non-nil
one wrapped in a frozenMap type.

Drop the frozen{List,Map,Message} types as no longer necessary.
(These types did have support for creating a read-only view of a
non-empty value, but we are not currently using that feature.)

Change-Id: Ia76f149d591da07b40ce75b7404a7ab8a60cb9d8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189339
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/convert_map.go b/internal/impl/convert_map.go
index 185d408..4182cbe 100644
--- a/internal/impl/convert_map.go
+++ b/internal/impl/convert_map.go
@@ -42,6 +42,10 @@
 	return c.PBValueOf(reflect.MakeMap(c.goType))
 }
 
+func (c *mapConverter) Zero() pref.Value {
+	return c.PBValueOf(reflect.Zero(c.goType))
+}
+
 type mapReflect struct {
 	v       reflect.Value // map[K]V
 	keyConv Converter