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>

jni/c/headers/mingw/jni_md.h

@@ -12,6 +12,8 @@
 #define JNIIMPORT __declspec(dllimport)
 #define JNICALL __stdcall
 
+#include <stdint.h>
+
 typedef long jint;
 typedef int64_t jlong;
 typedef signed char jbyte;