None 4-byte unsigned int data types

[shaomeng]

The current metadata packing mechanism relies on that unsigned int being 4 bytes. However, it might not be the case, as Peter puts it:
"I'm also moderately concerned about the use of type punning. Part of this is HDF's fault for using unsigned integers (which are not safe for this purpose; only unsigned char is), which need not be 32 bits wide (e.g., the LP32 and (S)ILP64 data models use 16 or 64 bits)."