Implement previousValue on EliasFanoReader

Implement previousValue on EliasFanoReader

Summary: It is often useful to retrieve the value preceding the current value
in an Elias-Fano iterator, for example when the list represents
adjacent ranges. This diff implements a new method `previousValue` in
`EliasFanoReader`.

It also adds a new `kUnchecked` boolean template argument to
`EliasFanoReader` which skips bounds checking.

Reviewed By: @philippv

Differential Revision: D2155049