Add support for action parameters with type varbit

[jafingerhut]

Attached to this comment: p4lang/p4-spec#264 (comment) is a small P4_16 program named try-varbit-action-parameter.p4.txt that gives no errors when compiled using p4test, but is not supported by the latest p4c BMv2 back end code as of 2019-Nov-11. I have not checked whether BMv2 contains the support that would be needed to execute it or not, but am creating this issue as a reminder to look for anything not supported in BMv2 that would be needed.

I know that the P4Runtime API v1.1 does not support varbit type action parameters, so P4Runtime API support would either have to run ahead of the P4Runtime spec to support it in BMv2, or more likely wait for that support in P4Runtime API v2.x

I do not know whether the Thrift API and/or simple_switch_CLI would be able to support such action parameters, but if they could, that would enable creating an STF test for p4c whenever its BMv2 back end is enhanced to support this construct.