Wallycore

- struct wally_operations has changed to hold the size of the struct
and has an additional member to allow overriding the lib secp context
used internally. Users must recompile their applications against this
version as a result (re-linking or simply upgrading the shared library
is insufficient).

- Build: Note that the secp256k1-zkp library is now a git submodule rather
than being directly checked in to the source tree. Run
`git submodule sync --recursive` then `git submodule update --init --recursive`
from the source tree in order to clone the secp source and build the library.
When you sync this change initially you may need to `rm -r src/secp256k1` then
`git checkout src/secp256k1` to remove any old files and achieve a clean
source tree.

- No API changes

- Python: 'None' passed as a binary buffer argument to wally calls which
require the buffer to be non-NULL now consistently throws ValueError (Just
as the library does for incorrectly sized or otherwise invalid inputs).
Previously this might throw a TypeError depending on the function.

- wally_is_elements_build now takes a size_t output instead of uin64_t.

- elements_pegout_script_from_bytes, asset_pak_whitelistproof and
psbt_to_bytes now follow the library convention for too-short buffers
instead of returning WALLY_EINVAL. See the generated API documentation
section "Variable Length Output Buffers" for details.

- FINGERPRINT_LEN was renamed to BIP32_KEY_FINGERPRINT_LEN for
consistency - You should change any references in your source when upgrading.

- Almost all functions comprising the PSBT interface have changed name,
arguments, semantics or all three. Users can consider the new interface
final for non-Elements PSBTs, however the Elements PSBT (PSET) interface
and implementation will be changed in the next release to match the
Elements codebase.

- Python 2 wheels are now deprecated. Users should move to Python 3 as soon as possible.

- API change of wally_asset_pak_whitelistproof to return the number of bytes written.