Changed nested table structures.

Plain debug BEAM tested successfully against the ets_SUITE.
Added nested and non-nested versions of common functions.

All the functions operating on LinearHashTable were rewritten into
two different versions: one version operating on RootDbTerm (nested)
and one version operating on DbTableNestedHash (non-nested).
Removed the LinearHashTable struct.

LinearHashTable's fields were moved into RootDbTerm and
DbTableNestedHash structs. Moreover, those moved into RootDbTerm
have no more atomic types, so some of the macro used to manage
access to those atomic fields were removed too.