hdf5_ep

[svn-r19758] Purpose:
    Improve h5diff performance.

Description:
    The following changes for improving h5diff performance:
    1) use HDmemcmp() before comparing each elements 
       (memcmp() is very fast at both linew and jam)
    2) replace the expensive H5Tequals() calls
    3) retrieve datatype information at dataset level not each element level 
       for compound datasets
    Merged from hdf5 trunk r19757.


Tested:
 jam (linux32-LE)