cmd/compile: add support for generic channels and type conversion during calls

Add support for channels in subster.typ(). Add new test file chans.go.

To support assignability of bidirectional channel args to directional
channel params, I needed to type check generic calls after they are
instantiated. (Eventually, we will create separate functions to just do
the assignability logic, so we don't need to call the old typechecker in
this case.) So, for generic calls, we now leave the call as OCALL (as a
signal that the call still needs typechecking), and do typecheck.Call()
during stenciling.

Smaller changes:
 - Set the type of an instantiated OCLOSURE node (and not just the associated
   OFUNC node)

 - In instTypeName2, filter out the space that types2.TypeString inserts
   after a common in a typelist. Our standard naming requires no space
   after the comma.

 - With the assignability fix above, I no longer need the explicit
   conversions in cons.go.

Change-Id: I148858bfc6708c0aa3f50bad7debce2b8c8c091f
Reviewed-on: https://go-review.googlesource.com/c/go/+/301669


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>