Introduce Schnorr signature

[kseo]

It's ready now.

[kseo]

Some preparation patches were landed: 6359ab0, 9a78f45, c4f49a8, 37a34b8, 418d472, 94ef1e6, 0990b70, 6e78e95

[hyunsikjeong]

To-do

• Read the issues in libsecp256k1 which are related to Schnorr signature.
• Make a demo for Schnorr signature.
• Implement Schnorr signature in the CodeChain primitives and the SDK.

[hyunsikjeong]

Finished working on the primitives.

[hyunsikjeong]

Finished working on CodeChain and the SDK.
CodeChain: https://github.com/CodeChain-io/codechain/commits/schnorr
codechain-sdk-js: https://github.com/codechain-io/codechain-sdk-js/tree/schnorr
codechain-keystore-js: https://github.com/CodeChain-io/codechain-keystore-js/tree/schnorr

[hyunsikjeong]

Might need to change the code of libsecp256k1. See https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki#cite_note-4, the current code uses the option 2.