refactor(bdk): use idiomatic naming for all getter methods

[notmandatory]

          I don't think this should be addressed in this PR, but I think we should use idiomatic naming for methods: https://rust-lang.github.io/api-guidelines/naming.html#getter-names-follow-rust-convention-c-getter

I.e. Wallet::get_address should be Wallet::address.

Originally posted by @evanlinjin in #1028 (comment)

[ValuedMammal]

Rename Psbt::get_utxo_for -> utxo_at_index ?

or get_txout

[LLFourn]

My personal preference which is slightly different from the rust style guide: I'm fine with get_ if there is something like a corresponding insert_. So get_tx feels good because there's also insert_tx. It shouldn't be get_balance though.

[notmandatory]

A small change but I think we should push to 2.0 milestone.