auto: Add Pick constructor to aregex for Choice
auto: rpc_branchy impl, replace rpc_par with it

rpc_branchy takes a branchy link-free constructor and a branchy regex
constructor, and does the recursion on two subexpressions using those.

This commit reimplement rpc_par in terms of rpc_branchy.
auto: implement Choice case in regex_of_policy

Uses rpc_branchy and appropriate constructors