1) Be sure to use the eval-time encoder/decoder rather than
the compile-time decoder.  In a few cases the receiver for
the compiling encoder was being pickled incorrectly into a
closure.
(This is the fix for issue 1238).

2) Get the innermost name right when given a pointer to an
unnamed type.

3) Use a count to delineate interface values, making it
possible to ignore values without having a concrete type
to encode into.  This is a protocol change but only for the
new feature, so it shouldn't affect anyone.  The old test
worked because, amazingly, it depended on bug #1.

Fixes #1238.

R=rsc, albert.strasheim
CC=golang-dev
https://golang.org/cl/2806041