Fix parsing of committed datatype metadata with h5wasm

[axelboc]

I was wrongly calling get_dataset_metadata instead of get_datatype_metadata on committed datatypes.

But get_datatype_metadata (and later get_attribute_names) had issues of their own, which are now fixed in https://github.com/usnistgov/h5wasm/releases/tag/v0.7.7 and https://github.com/usnistgov/h5wasm/releases/tag/v0.7.8

I'm also making use of the new Module#open and Module#close_file methods to avoid using the low-level ccall method, and I refactor the h5wasm demo to allow dropping a new file on top of the viewer (like in myHDF5) so I can make sure that the clean-up effects of the providers do call close_file as expected.