Changed a number of APIs using `isize` when `usize` is semantically more appropriate

[metasim]

• [X] I agree to follow the project's code of conduct.

• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

Scope:

• Driver::create.*
• Rasterband::overview
• Dataset::{layer|into_layer|layer_count}

Closes #495

[metasim]

@lnicola

should we also change rasterband and the histogram APIs to return an error?

My apologies, but I'm not following your question... 🤔

[lnicola]

We panic there if the index is too large.

[lnicola]

I meant https://github.com/georust/gdal/blob/master/src/raster/rasterband.rs#L39 and https://github.com/georust/gdal/blob/master/src/raster/rasterband.rs#L1010 etc.

[metasim]

ooooh, yeah... let me fix that. Given the option, I think passing the error up is better.

[metasim]

@lnicola Do I still have your approval after this additional change?

https://github.com/georust/gdal/pull/497/commits/75b4c122fbd2e6c487c6647f0bc8fe65ba08f321