Support calling ctors with tuples

[leighmcculloch]

What

Support calling ctors with tuples directly.

Why

This is a pattern we used with event publishing which also accepts tuples directly.

It allows a user to call:

env.register_contract_with_constructor(None, Contract, (a, b, c))

Instead of:

env.register_contract_with_constructor(None, Contract, (a, b, c).into_val(&env));