Fix abigen with int expression template arguments

[nathanielhourt]

Change Description

In my contracts, I use several template arguments which are types which resolve to integral values (such as an eosio::name) but are not trivially integer values. This works great in my contracts, but causes abigen to fail.

This PR fixes abigen in cases where a template argument is an expression that resolves to an integer, but isn't an integer literal.

Note: This is targeted at the master branch instead of develop because develop fails to build my contract due to other reasons. I'm hoping that this is just instabilities in unreleased code. In the meantime, I'm staying on 1.8.1, and so I base my PR there.

API Changes

• [ ] API Changes

Documentation Additions

• [ ] Documentation Additions