Refactor PSBT module & add new PSBT fields

[SachinMeier]

#42

Make PSBT use Derivation Path & Xpub modules

Add Taproot fields & PSBT v2 fields

Refactor some but not all odd logic

TODO:

• add_<field> funcs
• Fix some weird parse/serialize logic wrt list reversal
• add PSBT Role modules/funcs
• Write our own tests for new Taproot fields & v2 fields

[kafaichoi]

what's meaning of the prefix t for private function like tfrom_string, tto_bin?

[SachinMeier]

I named the recursive helper functions differently, but there's no real reason for that. I'll remove and make them all the same.

[philipglazman]

Same Q as @bruteforcecat. Otherwise, lgtm

[kafaichoi]

nit. it will be great to merge &serialize/1 and &serialize/2 into one &serialize/2 with type signature like

@spec serialize(t(), list({:with_checksum?,  boolean}) :: binary
  def serialize(xkey, opts \\ []) do
     with_checksum? =  Keyword.get(opts, :with_checksum?, true)
      extended_key_without_checksum_bin = (xkey.prefix <>
       xkey.depth <> xkey.parent_fingerprint <> xkey.child_num <> xkey.chaincode <> xkey.key)
    case with_checksum? do
       true ->
          Base58.append_checksum(extended_key_without_checksum_bin)
        false ->
           extended_key_without_checksum_bin
    end
  end

I think it's more elixir-idiomatic, better typing for user using dialyzer and less code duplication

[kafaichoi]

tACK. one nit and we need to fix merge conflict

[SachinMeier]

move split & reverse into path_from_string,