feat: interceptor based transformers (#122)

See the idea surfaced in #114.

This commit introduces interceptor based transformers by using the
`:enter` and `:leave` keys returned on the `-value-transformer`
function.

Naively it uses almost identical implementations for all enter and exit
applications, with the only exception being the collection-based
transformers which do look at the phase to avoid double wrapping
collections.

Ultimately there might be some occassions where we could speed things up
even more (eg. not reducing over a collection if there isn't a mapping
function anyway) but those optimizations don't exist in the current
implementation either, so this PR doesn't attempt to resolve any of
that.