Clarify jni_x_call macro names

[dmitry-timofeev]

Clarify the naming to express behaviour more precisely:

• jni_non_null_call must probably be jni_non_void_call — as it does not check its return value to be non-null
• jni_call must be jni_non_null_call — as it works for pointer/references only and checks its return value to be non-null.
• Consider using a separate macro for exception-safe methods.
• not_null → null_to_err, see the discussion: Further improvements of macro usage for JNI calls  #150 (comment)

[dmitry-timofeev]

I assume the macros are now a part of the public API of the crate:

jni-rs/src/lib.rs

Lines 208 to 209 in d4ed1b3

	#[macro_use]
	mod macros;

Shan't we make it private?

[stanislav-tkach]

I see no reason to make these specific macros public.

[dmitry-timofeev]

I see no reason to make these specific macros public.

Yeah, that's rather an implementation detail — and if some JNI methods are missing, it probably makes sense to add them in this library.

[dmitry-timofeev]

Updated the description

[dmitry-timofeev]

I've mistaken the definition — these macros aren't actually exported 🙃