the index mapping contains some relatively messy logic
and the top-level bleve package only cares about a relatively
small portion of this
the motivation for this change is to codify the part that the
top-level bleve package cares about into an interface
then move all the details into its own package
NOTE: the top-level bleve package still has hard dependency on
the actual implementation (for now) because it must deserialize
mappings from JSON and simply assumes it is this one instance.
this is seen as OK for now, and this issue could be revisited
in a future change. moving the logic into a separate package
is seen as a simplification of top-level bleve, even though
we still depend on the one particular implementation.
79cc39a67e