Move C API to submodule

[simonbyrne]

This moves all the C API functions and types to an API submodule (this was part of #834, but changed to not depend on #833).

The tests aren't yet passing as h5d_read_chunk in src/api/helpers.jl depends on get_chunk_length and get_chunk_index (defined in src/api_midlevel.jl). It doesn't really make sense to have the C API depend on higher-level API functions, so one of these should probably be moved: which would make more sense?

[simonbyrne]

h5d_read_chunk in src/api/helpers.jl depends on get_chunk_length and get_chunk_index (defined in src/api_midlevel.jl). It doesn't really make sense to have the C API depend on higher-level API functions, so one of these should probably be moved: which would make more sense?

Given that these functions perform allocations and use keyword arguments, I decided it made more sense to move them to the midlevel (and deprecated some function signatures which were a bit confusing and unnecessary).

[simonbyrne]

I think this is ready. @musm any thoughts?

[simonbyrne]

One question is whether we should move src/error.jl into src/api/? That's the only place where the lower-level API accesses a higher one.

[musm]

It's a good question. I was thinking the same, for the reason you mention. The question is whether we should use H5Error for all exceptions in the HDF5 and API module, if that's the case then we have to keep it as is. If we want to restrict the error to the inner API, , perhaps we should create an H5LibError <: H5AbstractError as a subtype?

[musm]

Hmm, on second thought, I'm actually thinking it would be better to leave the custom exception handling in the API module, since it is strictly being used to handle internal hdf5 exceptions and wouldn't make sense to use outside.

[musm]

Let me know what you think about the last commit.

Given that the error is isolated in its own API module., we could even reuse the same H5Error name in the upper level module HDF5 without issues, as a higher level Exception to deal with errors in that module, whereas HDF5.API.H5Error makes it clear it handles the internal API level errors.

[musm]

I used typeof here so that the full module path is printed:

I think this is a good idea in case we decide to use the same exception error name in the main module.

[musm]

I think this PR is good to go, unless you think we should handle the internal errors in a different manner.

[simonbyrne]

LGTM