Rename duk_hobject e_used to e_next for clarity

e_used is actually used to track the number of entry part entries currently
GC reachable.  Some of the entries may be NULL, indicating deleted keys.
The e_used value is also used as the index where the next insertion of a new
property key happens, so e_next is probably a more appropriate name and
doesn't incorrectly imply that the e_used count indicates actually used,
non-NULL key entries.
e_next rename, FIXME note for Duktape.info()
Merge branch 'rename-eused-enext'
Clarifying comment for Duktape.info() e_next
Rename e_used to e_next in guide, clarify
Guide Duktape.info() tweaks
Merge branch 'duktape-info-eused'