Feature request: make Handle public

[Berrysoft]

I'm working on wrapping HDF5 High-level APIs into safe Rust: https://github.com/Berrysoft/hdf5-ext/tree/master/hdf5-hl ; see #224 .

Now it works well, but it would be better if Handle and other low-level APIs (for example, ObjectClass) could be made public. Then I can remove some transmute affairs.

[mulimoen]

You can set path in Cargo.toml in your fork and then we could discuss if this is wanted when you make the PR

[Berrysoft]

I reconsidered and found that it would be more flexible if I maintain the hdf5-hl myself. Therefore I'm only asking to make some apis public. After the crate becomes mature, I will consider to open a PR.