Centralize uses of MNumeric.BIGINT for internal index fields.

When mcompat is deprecated, this can change to the standard bundle. Even if the schema is still using mcompat, since these fields are not exposed to the query itself but only used internally for unique index null separators and Z-order spatial encoding and so on.

This has the same motivation as the types translator, although it does not depend on it.

Depends on previous pull requests. If there is an easier way to submit these stacked branches, let me know.