[jnigen] `JniX` to `JX` rename (https://github.com/dart-lang/jnigen/issues/126)

Closed https://github.com/dart-lang/jnigen/issues/124.
Closed https://github.com/dart-lang/jnigen/issues/125.

* Added `JPrimitive` and its subclasses instead of using the typedefs from `ffi` directly.
* Renamed `JniObject`, `JniString`, and ... to the shorter `JObject`, JString`, and ...
* Made tests more robust by deleting the objects in an arena.
38 files changed
tree: 372100b7aae679a0c45edbc8740beecef4f24015
  1. .github/
  2. pkgs/