hdf5_ep

[svn-r12575] Description:
    Allow the heap ID length to be chosen at heap creation time, to allow
for making heap IDs long enough to directly embed the file offset & length
of 'huge' objects in the heap ID (which allows them to be retrieved directly
from disk, instead of requiring them to be looked up in the B-tree that tracks
'huge' objects)

Tested on:
    FreeBSD/32 4.11 (sleipnir)
    Linux/64 2.4 (mir)
    Solaris/64 9 (shanti)