Add support for musig2 (#93)

* Use Jonas Nick's musig2 branch

* Reformat c code (no functional changes)

* Implement musig2

* Add documentation to musig2 functions (#97)

Usage of the Musig2 functions isn't intuitive at all, especially with
the key aggregation cache and session data. It's important to provide
accurate documentation to help users understand how to correctly produce
musig2 signatures.

We also change argument names to match Kotlin best practices instead of
using the same argument names as C functions.

* Add musig2 reference tests (no functional changes)

---------

Co-authored-by: Bastien Teinturier <[email protected]>

Author: sstone

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "native/secp256k1"]
 	path = native/secp256k1
-	url = https://github.com/bitcoin-core/secp256k1.git
+	url = https://github.com/jonasnick/secp256k1.git

build.gradle.kts

@@ -22,7 +22,7 @@ buildscript {
 
 allprojects {
     group = "fr.acinq.secp256k1"
-    version = "0.14.0-SNAPSHOT"
+    version = "0.14.0-MUSIG2-SNAPSHOT"
 
     repositories {
         google()