In the next patch we are going to teach _selcase to support both
external and inplace data. Before that let's do a couple of preparatory
things.

This patch: introduces .ptx() and .prx() accessors to get to
corresponding data buffer associated with _selcase. Convert _selcase
users to use .ptx() and .prx() instead of doing direct .ptxrx access.
This way when we'll add inplace support to _selcase, we'll need to adapt
only accessors, not clients.

The only place that is left using .ptxrx directly is one tricky place in
_chanselect2<onstack=false> where gevent-related code needs to carefully
deal with proxying tx/rx buffers due to STACK_DEAD_WHILE_PARKED.

NOTE even though new accessors may panic, libgolang.cpp always calls them
after checking that the conditions for ptx/prx calls are valid.