Heads up for planned jni-sys updates

[rib]

Wasn't sure whether to open a discussion for this, but essentially just wanted to make sure to give a heads up here that I've been chipping away at some maintenance chores for jni-sys and based on issues I've seen while working on the jni crate I was planning a few semver breaking changes.

It'd be good if you can take a look at the 'Release 0.4' milestone here: https://github.com/jni-rs/jni-sys/milestone/1

The two notable changes are:

1. making jboolean an alias for bool: ref Considering redefining jboolean as either an bool or our own #[repr(u8)] enum jni-rs/jni-sys#19, Add Array wrapper types with lifetimes jni-rs/jni-rs#400 (comment) and Make jboolean an alias for bool instead of u8 jni-rs/jni-sys#23
2. Turning JNINativeInterface + JNIInvokeInterface_ into unions that namespace function pointers by the JNI version they were added (and remove the misleading Option wrappers), ref: Turn JNINativeInterface + JNIInvokeInterface_ into unions jni-rs/jni-sys#28 and Removes the Options around the function pointers jni-rs/jni-sys#25

The latter change in particular will likely involve boilerplate changes in duchess but at least as far as the jni crate goes I'm glad I looked at this this since it has highlighted several places where the jni crate was using > 1.1 functions without checking the version at runtime.