Processing `H5T_VLEN` values

[axelboc]

@bmaranville I think I remember you mentioning that you had already investigated reading H5T_VLEN datasets and that it wasn't trivial. I'm interested in giving it a go myself. Any pointers you could give me?

In the H5Web repo, I generate a sample.h5 file with a few H5T_VLEN datasets: /vlen_int8_scalar, /vlen_int64_1D, and /vlen_utf8_1D. You can see how h5grove (i.e. h5py) reads them: https://h5web.panosc.eu/h5grove?file=sample.h5 versus how h5wasm currently reads them: https://h5web.panosc.eu/h5wasm?url=https%3A%2F%2Fwww.silx.org%2Fpub%2Fh5web%2Fsample.h5

h5grove	h5wasm