Start enabling -Wextra and -Wconversion in "rn_xplat_cxx_library" (1/2)

Summary:
## Stack

These can suss out some real bugs, and helps further avoid mismatch with downstream MSVC on /W4 as used by MSFT.

I enabled the families of warnings, but suppressed some major individual warnings that weren't clean. But I did clean some up, notably, missing initializer, and shortening 64 bit to 32 bit. We can do some of the rest incrementally (e.g. `-Wunused-parameter` has a fixit).

This change illuminates that MapBuffer is missing 64 bit integer support, but we often pass 64 bit counters to it, which is a bug. For now I just left TODOs around those.

`rn_xplat_cxx_library` is used for external libraries interfacing with RN, which we probably don't want to police, so I structured these stricter warnings as an opt-in flag, only enabled for our own rules.

## Diff

This fixes up source code to avoid emitting the extra warnings now enforced. Of what is enabled, this is mostly shortening 64 to 32, or missing field in initializer.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D52589303

fbshipit-source-id: 11cb778d065799fd0ead3ae706934146d13500bb

cxx/fbjni/detail/CoreClasses-inl.h

@@ -425,7 +425,8 @@ auto JArrayClass<T>::newArray(size_t count) -> local_ref<javaobject> {
   static const auto elementClass =
       findClassStatic(jtype_traits<T>::kBaseName.c_str());
   const auto env = Environment::current();
-  auto rawArray = env->NewObjectArray(count, elementClass.get(), nullptr);
+  auto rawArray = env->NewObjectArray(
+      static_cast<jsize>(count), elementClass.get(), nullptr);
   FACEBOOK_JNI_THROW_EXCEPTION_IF(!rawArray);
   return adopt_local(static_cast<javaobject>(rawArray));
 }
@@ -434,7 +435,9 @@ template <typename T>
 inline void JArrayClass<T>::setElement(size_t idx, T value) {
   const auto env = Environment::current();
   env->SetObjectArrayElement(
-      this->self(), idx, detail::toPlainJniReference(value));
+      this->self(),
+      static_cast<jsize>(idx),
+      detail::toPlainJniReference(value));
 }
 
 template <typename T>
@@ -625,7 +628,12 @@ inline void PinnedPrimitiveArray<T, Alloc>::abort() {
 template <typename T, typename Alloc>
 inline void PinnedPrimitiveArray<T, Alloc>::releaseImpl(jint mode) {
   FACEBOOK_JNI_THROW_EXCEPTION_IF(array_.get() == nullptr);
-  Alloc::release(array_, elements_, start_, size_, mode);
+  Alloc::release(
+      array_,
+      elements_,
+      static_cast<jint>(start_),
+      static_cast<jint>(size_),
+      mode);
 }
 
 template <typename T, typename Alloc>