Add some E_let_tuple commuting conversions

These are analogous to the existing E_let commuting conversions, but for E_let_tuple.

This makes some examples optimise better.

For example: https://github.com/pewulfman/Tezos-TZIP-implementation/blob/0db4dca55a86ac1af7b0bd78fefa99381477a36b/TZIP-5%20(FA1)/FA1.mligo

Before this change, this example did not compile successfully, because some lambdas which should have been inlined and beta-reduced away could not be, and so were compiled to illegal closures (closing over big_map or contract.)