Add From trait for &BitcoinTxIn

[thunderbiscuit]

This trait is required for our use of the TxIn type.

I also:

• Made a small clean up commit to use what I think is easier to read variable names for the From trait
• Made the return type of the From trait explicit instead of using Self (my understanding is that it's more idiomatic that way).

[reez]

• Made the return type of the From trait explicit instead of using Self (my understanding is that it's more idiomatic that way).

Good to know that's the more idiomatic way, I'll have to keep that in mind-

[reez]

ACK 165f910

(My assumption is the Failing Kotlin CI test will be resolved after #26 gets in)

[rustaceanrob]

For future reference, I am not sure I would go about implementing this From conversion. It abstracts away a script_buf.clone() that ideally we should have to call explicitly higher up the stack IMO. If I recall correctly from bdk-ffi, this is used when calling iter on vector of TxIn. Instead, I think you would want to call into_iter on a cloned Vec<TxIn>, which takes ownership of each of the TxIn. So the cost of cloning is more visible higher up the stack