Deprecate get_ prefix for Block and Transaction methods

[shesek]

To match the changes introduced in rust-bitcoin/rust-bitcoin#861.

This allows using unified code with the same methods for both elements::{Transaction,Block} and bitcoin::{Transaction,Block} (behind different features), without triggering rust-bitcoin's deprecation warnings for the get_-prefixed methods.

Note that there are other instances of get_-prefixed methods in the rust-elements codebase. I only updated the ones needed for compatibility with the changes made in rust-bitcoin.

[shesek]

I ran into this in electrs, where I ended up creating an EBCompact trait to make the get-less methods available on the rust-elements data structures.

Another incompatibility that I ran into is that rust-bitcoin (as of v0.28) iterates over witness elements as a &[u8], while rust-elements iterates them as a &Vec<u8>.

(Initially I had to use Vec::from instead of Clone for compatibility with rust-bitcoin v0.28, then made it conditional to support rust-elements which still requires using Clone)

[shesek]

This assumes that the next release is going to be 0.19.1, which might not be the case.

[apoelstra]

ACK 2a35bad

[apoelstra]

I'm happy to do a quick point release if you want. There's nothing else in the queue.

[shesek]

No urgency for me, I can keep using the compatibility trait until a new release is out. Thanks for merging!