Add JNA 3rd party library.
Stub native call ops.
Stub the NativeCall and CPointer REPRs.
Set correct argument signatures for native call ops.
Add JNA to build and runtime JARs.
First implementation of nqp::buildnativecall.
Move constants to enum from static final byte FOO.
Build NativeCallInstance.{arg_types,ret_type} in nqp::buildnativecall.
Basic call handling and converting between NQP and JNA types.

This means that we support int, num, str and cpointer as arguments and return
values, which in turn means that we pass the basic sanity tests in the NQP
test suite.
Run native call tests as part of the standard test target.
Merge branch 'master' into nativecall-jvm
Stub the CArray and CStr REPRs.

We need these to get NativeCall.pm6 to compile.
Stub nqp::nativecallrefresh.

Also needed to get NativeCall.pm6 to compile.
Merge branch 'master' into nativecall-jvm
Some NativeCall error handling tweaks.
Do "current process" loading in a portable way.
NativeCall test tweak; now passes on Windows.
Hold native callsite info at an extra indirection.

This will help support inlining it into a P6opaque.
Teach NativeCall REPR how to inline itself.

Note that there's not currently a way to get hold of the inlined thing
so NativeCall can use it; that will come next.
Install jna JAR.