Remove coincurve dependency, use python-bitcointx

[AdamISZ]

This commit changes the underlying functions used in
jmbitcoin from the private and public key primitives
in coincurve and replaces them with equivalent
primitives CKey and CPubKey from python-bitcointx,
this removes the need to install coincurve and its
own bundled libsecp256k1 dynamic library.
Note that an additional pubkey_tweak_mul function
is exposed with ctypes from python-bitcointx's
bundled libsecp256k1 library.

[AdamISZ]

This is marked [WIP] while a few things still need to be done:

• Remove coincurve from the setup.py of jmbitcoin
• Remove the libffi build dependency in install.sh (unless I forgot something, this is only needed for coincurve) edit: oh bad memory, it was originally put in place for libsodium, so that won't be removed.
• Test installation is still functioning correctly
• Cross-compatibility tests of old versions of joinmarket with ones using this version (specifically to make sure that the inter-participant message sign/verify is compatible, but also other functions).

[AdamISZ]

As of 537e317 I'm now happy with merge of this.
Tested installation; tested cross compatibility of coinjoin in both directions, i.e. with taker side being this branch and maker side being master branch, and then vice versa.

Will do a few more tests of minor functions, i.e. various different wallet methods and payjoin, before actually merging.

Other review and test contributions would be very much appreciated!

[AdamISZ]

Have now tested all the relevant wallet-tool methods (i.e. ones involving keys or transactions somehow), and payjoin between a master-branch receiver and a this-branch sender (which also tests the PSBT signing code under the hood). Everything seems to be working correctly.

[kristapsk]

Concept ACK on removing extra dependencies. Will do some testing.

[kristapsk]

ACK 537e317. Didn't notice any problems in my tests.

[AdamISZ]

Thanks for review, merging, will probably add #1098 after some more checking and testing.