…of leaf types

Addresses an off-by-one in the type lookup, ensures we look at at all
entries when doing invalidation, and allows putting any Type{T} object
also in the cache (I forgot these always had `simplesig`, so we needed
to handle that case too to receive the benefit for them).

If the environment is trivial (empty) and the type is a dispatch tuple,
we know the intersection result is trivial, and can skip it.

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>