Simplify match token_branch in funding_path.

[roconnor-blockstream]

The

match token_branch {
    Left(u: ()) => assert!(jet::eq_32(jet::current_index(), 0)),
    Right(u: ()) => assert!(jet::eq_32(jet::current_index(), 1)),
};

construct in funding_path can be simplified to asserting that the current index is ≤1 or alternatively <2.

[roconnor-blockstream]

cc @apoelstra

[KyrylR]

Addressed the issue here: #10