Implement Partially-signed Elements transaction support

[dgpv]

The spec is in progress at ElementsProject/elements#900

A note on naming:

I find PSET confusing when used in implementation. The implementation will reuse a lot from PSBT, and having the abbreviation differ just by one letter in the middle, and the letters very similar will create a lot of confusion.

Because of this, I will use PSBT_Elements prefix for names in python-elementstx. Of course this clash comes from that I used PSBT prefix rather than BIP174 prefix. But PSBT is just much more ergonomic for names when BIP174, and I strive for this lib to have ergonomic interface.

I will just say that PSBT stands for 'partially signed bitcoin-like transaction'.