☂️ Consider making certain JNI routines leaf functions

[HosseinYousefi]

For some of the routines, we can generate both the normal and the leaf versions, and call each one depending on the situation. For example Set<PrimitiveType>ArrayRegion could be fast depending on the length of the region, maybe it makes sense to conditionally call one or the other based on this.

Candidates:

• [ ] FindClass
• [ ] DeleteGlobalRef
• [ ] Get<Member>ID
• [ ] GetStringLength
• [ ] GetArrayLength
• [ ] IsInstanceOf
• [ ] Primitive array operations
• [ ] Direct byte buffer operations
• [ ] String methods – especially when https://github.com/dart-lang/sdk/issues/39787 is landed

For jnigen, users should be able to specify if a method is leaf or not, this can be done as a part of jnigen transformation. However simply:

• [ ] Getting a field
• [ ] Setting a field Can be safely turned into leaf calls.

cc/ @dcharkes