Add conversions for JObject wrapper types

[uvlad7]

No description provided.

[uvlad7]

@giovanniberti I also added a fix for Box<[bool]> conversion here.

And I need your advice how to add tests for unchecked conversion too.

[uvlad7]

I implemented Object[] <=> Box<[JObject<'env>]> conversion, but I don't know how to make the following

       #[call_type(unchecked)]
        pub extern "jni" fn userArray() -> Box<[User<'env, 'borrow>]> {
            vec![].into_boxed_slice()
        }

work, because it's not possible to impl foreign trait for slices and it's also not possible to impl Signature for Box<[T]>.

@giovanniberti can you take a look, please? I really need this feature to implement some bindings (it actually doesn't matter if rust type is Box<[T]> or something else, but I need to be able to accept arrays as parameters and return them from rust)

UPD: I created conversions for one-dimensional arrays, but not for other types. I'm wondering how to impl TryFromJavaValue for structures like struct UserArray(Box<[User]>) where From<Box<[User]>> for UserArray is implemented. Obviously, in that case we can construct UserArray from java value, but I don't understand how to implement this.

UPD2: I created an example in the tests, but I think it'd be better to impl it with derive for such a wrapper types, and without constcat dependency and this additional ArrSignature trait.

[uvlad7]

@EliseChouleur So, I assume it's better to wait for #59, right?

[uvlad7]

@giovanniberti , @EliseChouleur , take a look, please. There are still a lot of improvements to be added, but I assume it'd be better to merge this to fix bugs and to avoid conflicts in migration to jni-0.21, since this PR is really huge.

[giovanniberti]

Sorry for the very late review. (understatement of the century right there :) )
First of all: impressive work! You did manage to upgrade jni to 0.21, I tried that a while ago and gave up because of the API changes

Just a few comments:

1. There are a few commented lines here and there, I'd eliminate those if not needed
2. There are a couple new traits related to array handling but they don't seem to be documented, could you add a few lines on those?

Thank you again for all the effort!

[uvlad7]

Uh, sorry, I created a mess with branches, and unfortunately I didn't manage to update jni to 0.21.1, moved to another branch and gave up. This MR contains other useful fixes, though, and I can return it back into working condition - with jni 0.20 as a dependency - if you want to merge.
I want to continue my efforts in migration to 0.21, but I won't have any time for that in the nearest future

[giovanniberti]

This MR contains other useful fixes, though, and I can return it back into working condition - with jni 0.20 as a dependency - if you want to merge.

Sure!

I want to continue my efforts in migration to 0.21, but I won't have any time for that in the nearest future

No problem, I myself have left this repo in a semi-abandoned state since last year — it's in the spirit of FOSS that people can contribute whenever they want and can :)

[uvlad7]

I reverted stuff back to jni 0.20.0 version, looks like it's ready to be merged.