Implement `inner`/`loginner` for TTN in terms of `contract`

[mtfishman]

I think ultimately inner/loginner for TT should just be designed based on calling a more general contract function with a contraction sequence appropriate for a tree network, and we can use a tree traversal as a way to quickly derive a reasonable sequence, and users can pass their own contraction sequence or function for computing a contraction sequence.

Originally posted by @mtfishman in #18 (comment)