optimization: define the process method at handler init time
optimization: define the process method at handler init time

In most cases a wrapping reducer's handler's process method takes a
single argument, but we keep_indexed requires 2, and custom reducers
might require more. This change supports the built-in cases efficently
and still allows for building custom transducers that can accept and
build handlers from a block w/ arity > 2.

cognitect-labs/transducers-ruby,cognitect-labs/transducers-ruby