Native jni build (#5)

* Each native library is in its own jar. Got rid of cross compilation. The project is `secp256k1` and not `secp256k1-kmp`.

* Updated CI to use all 3 OS VMs

Co-authored-by: Salomon BRYS <salomon@kodein.net>

gradle.properties

@@ -8,6 +8,7 @@ kotlin.incremental.multiplatform = true
 kotlin.parallel.tasks.in.project = true
 #kotlin.mpp.enableGranularSourceSetsMetadata = true
 kotlin.native.enableDependencyPropagation = false
+kotlin.native.ignoreDisabledTargets = true
 
 # https://github.com/gradle/gradle/issues/11412
 systemProp.org.gradle.internal.publish.checksums.insecure = true