add Bitcoin SPV wallet

[buck54321]

Adds support for a Bitcoin SPV wallet to the decred package. The wallet is implemented by wrapping btcwallet and compiling as a C library, and importing into Python using ctypes.

Since we're mostly concerned with faithfully transmitting data between the caller and btcwallet, our role in testing is straight-forward.

Before running tests (with poetry run pytest tests/integration/btc/ from the decred directory), build the library with either ./build-nix.sh or ./build-win.sh from the libbtcwallet directory.

You need to have Go installed to compile the library. Not certain yet how this translates to production.