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>

settings.gradle.kts

@@ -6,12 +6,16 @@ pluginManagement {
         jcenter()
     }
 }
-rootProject.name = "secp256k1-kmp"
+rootProject.name = "secp256k1"
 
 include(
     ":native",
     ":jni",
     ":jni:android",
     ":jni:jvm",
+    ":jni:jvm:darwin",
+    ":jni:jvm:linux",
+    ":jni:jvm:mingw",
+    ":jni:jvm:all",
     ":tests"
 )