Fix initial_state/0 implementation
Update Makefile to autogen lets_impl* files from hets_impl* files

The Erlang source, C++ header, and C++ source files for the LevelDB
and HyperLevelDB nif and driver implementations are identical except
for a few mechanical differences.  The Makefile using a simple,
bruteforce recipe automatically mirrors the HyperLevelDB nif and
driver implementations to the corresponding LevelDB implementations.
Reimplement key and object iterators as batch operations
Refresh edocs
Update rebar (2.4.0-3-ga00f8c6)