No ability to close new data types

[grimbough]

When we create a new data type with H5T_create or H5T_copy e.g. https://github.com/grimbough/rhdf5/blob/74982e942b00594112e36f99b7b2fda819866b26/R/h5create.R#L54

there's no way to keep track of the newly created datatype. We are returned an ID, but it isn't added to the list of open handles and is never closed.

Spotted when reviewing #80 & #79