Dynamically load libjvm rather than requiring it at build time

[marcprux]

This PR removes the build/link time requirement for libjvm and moves it into dlload/dlsym calls from JavaVirtualMachine.shared(). This greatly simplifies the Package.swift build by eliminating all the tool calls and path searching, and enables swift-java-jni-core to be a build dependency in environments that do not have any JVM installed. For example, it can be a transitive dependency for iOS libraries, where it won't actually work if you tried to use it, but it will build.

Since <jni.h> is no longer needed, we declare a minimal subset of the function table and types described in the JNI specification right into CSwiftJavaJNI.h. The only symbols we need to dynamically load and invoke are JNI_GetCreatedJavaVMs and JNI_CreateJavaVM; everything else falls out of the function tables.

Tested on macOS (with swift test) and an Android 28 emulator (with skip android test).

[ktoso]

That's an interesting idea, I think this might work? Thanks for looking into it Marc

[ktoso]

Refs #3

[ktoso]

I think we're happy with this, as this is derived from the spec and we don't copy the actual jni.h anywhere this makes sense we think. Thanks for pouring in the work and idea @marcprux !