Add pull/get_page API

[Saafo]

There are few things I'm not sure yet:

• I'm not sure whether we should use Vec::from_raw_parts for higher performance (I encountered with panic pointer being freed was not allocated, seems they don't use the same allocator?)
• Not sure std::ptr::drop_in_place would free the memory allocated by C, should we add a dependency libc to use libc::free?
• Whether we can add an examples/ or tests/ besides the readme

Hi @d-k-bo, do you have time to review this PR? I wish we can have a release when this PR is merged.

[Saafo]

The two issue about freeing pointer is solved: the pointer returned by ope_encoder_get_page doesn't need to be freed since it's a slice.

[d-k-bo]

Sorry for my late response. I was on vacation and now I am pretty busy preparing for an upcoming exam. It might take a while until I can look into this.

[Saafo]

Good suggestions! Fixed and request review again ❤️

[d-k-bo]

Whether we can add an examples/ or tests/ besides the readme

While it would be great to have proper unit tests (for the entire library), an example that shows how this API can be used would be a really good idea (either as an example in examples/ or as part of the docs for Encoder::create_pull).

[Saafo]

@d-k-bo I'm afraid that my example/test code is not ready yet. Can we merge this pr first?

[d-k-bo]

Thanks for your contribution!

[d-k-bo]

It is now available in version 0.2.2 on crates.io.