Allow primitives to assign to multiple results

[martijnbastiaan]

Approach:

-- A multi result primitive assigns its results to multiple result variables
-- instead of one. Besides producing nicer HDL it works around issues with
-- synthesis tooling described in:
--
--   https://github.com/clash-lang/clash-compiler/issues/1555
--
-- This transformation rewrites primitives indicating they can assign their
-- results to multiple signals, such that netlist can easily render it.
--
-- Example:
--
-- @
-- prim :: forall a. a -> (a, a)
-- @
--
-- will be rewritten to:
--
-- @
--   \a0 -> let
--            r  = prim a0 r0 r1     -- With 'Clash.Core.Term.MultiPrim'
--            r0 = multiPrimSelect r0 r
--            r1 = multiPrimSelect r1 r
--          in
--            (r0, r1)
-- @
--
-- Netlist will not render any @multiPrimSelect@ primitives. Similar to
-- primitives having a /void/ return type, /r/ is not rendered either.
--
-- This transformation is currently hardcoded to recognize tuples as return
-- types, not any product type. It will error if it sees a multi result primitive
-- with a non-tuple return type.
--