lib: add TxIn

[rustaceanrob]

Nothing too wild here. This mirrors what is going on in bdk-ffi and also adds the conversion from TxIn to BitcoinTxIn. Weirdly From<u32> for Sequence is not implemented otherwise this would be super clean.

[reez]

using the try_unwrap, looks good to me!

Good callout on:

Weirdly From<u32> for Sequence is not implemented otherwise this would be super clean.
... that is interesting and would end up looking clean.

[rustaceanrob]

Rebased. I also moved any std imports to be grouped, and any pub reexports to be grouped

[tnull]

This unfortunately needs another rebase now that #11 landed.

[rustaceanrob]

Rebased

[rustaceanrob]

Given the conversation on #16, I removed the Arc optimizations in favor of cloning.

[thunderbiscuit]

ACK 2e3fd74.

I'm not 100% certain yet we should not have the optimization, but I do agree it's worth asking on the uniffi repo and simply adding the optimization to all types that can make use of it once a final decision has been reached. In the meantime this means we can merge those quicker.