Taproot signing support

[GambolingPangolin]

This change adds the ability to sign taproot inputs, but behind a flag. This depends on changes from haskoin/secp256k1-haskell#40 , which also includes a nix shell definition to use to explore the changes. Cabal users can include the following cabal.project.local file build things with the appropriate flags.

packages:
    ./

source-repository-package
    type: git
    location: https://github.com/GambolingPangolin/secp256k1-haskell.git
    tag: origin/bip340-support

package haskoin-core
    ghc-options: -Wall
    flags: +bip340

package secp256k1-haskell
    ghc-options: -Wall
    flags: +bip340

[GambolingPangolin]

Closes #405