Make `begin` as the first index of a `Dataset` work

[machakann]

Consider that d is a Dataset with two dimensions. Currently, d[:, end] works, however, d[:, begin] doesn't. This PR intended to make the latter work.

using HDF5
fn = tempname()
fid = h5open(fn, "w")
g = create_group(fid, "foo")
d = create_dataset(g, "bar", Int, (1, 3))
d[1, 1:3] = 1:3

@show d[1, end]  # OK
@show d[1, begin] # Error

The error message is:

MethodError: no method matching axes(::HDF5.Dataset, ::Int64)
Closest candidates are:
  axes(::Base.Broadcast.Broadcasted{<:Any, <:Tuple{Vararg{T, N}} where T}, ::Integer) where N at broadcast.jl:227
  axes(::Number, ::Integer) at number.jl:83
  axes(::AbstractArray{T, N}, ::Any) where {T, N} at abstractarray.jl:72
  ...

Stacktrace:
 [1] firstindex(a::HDF5.Dataset, d::Int64)
   @ Base .\abstractarray.jl:402
 [2] top-level scope
   @ show.jl:1047
 [3] eval
   @ .\boot.jl:368 [inlined]
 [4] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
   @ Base .\loading.jl:1428

[machakann]

Additionally, this is optional but the definition of lastindex is no longer needed, probably. Base.lastindex also uses axes to work.

https://github.com/JuliaIO/HDF5.jl/blob/205518d6c1d09dc8586b577d88a2f3633a9ea061/src/datasets.jl#L134-L135

[musm]

Julia 1.0/1.3 have test failures

┌ Warning: Deprecated syntax `"begin" inside indexing expression` at /home/runner/work/HDF5.jl/HDF5.jl/test/extend_test.jl:26.

might need to version out the tests and or changes.

[musm]

Additionally, this is optional but the definition of lastindex is no longer needed, probably. Base.lastindex also uses axes to work.

If we don't need them, better to remove them for simplicity than cary uneccessary definitions.

[machakann]

Thank you for your comments. I will work on it. It seems I still need some tweaks for tests.

I found that the test failed without the two lines of lastindex in julia v1.4.2 and v1.5.4. So, the two lines should be kept for v1.4 and v1.5. Probably, https://github.com/JuliaLang/julia/pull/38742 is required to remove them. I will just add a comment on it. (begin just works without explicit firstindex definitions.)

[musm]

Does the @static need to be pulled out of the testset ? Seems like it's not being respected

[machakann]

Finally, all tests passed. Since begin is interpreted as a begin-end block in v1.3, I separate the file to avoid LoadError.

[musm]

Great thanks!