Add redstone relay block (#2002)

- Move redstone methods out of the IAPIEnvironment, and into a new
   RedstoneAccess. We similarly move the implementation from Environment
   into a new RedstoneState class.

   The interface is possibly a little redundant (interfaces with a
   single implementation are always a little suspect), but it's nice to
   keep the consumer/producer interfaces separate.

 - Abstract most redstone API methods into a separate shared class, that
   can be used by both the rs API and the new redstone relay.

 - Add the new redstone relay block.

The docs are probably a little lacking here, but I really struggled to
write anything which wasn't just "look, it's the same as the redstone
API".

doc/events/redstone.md

@@ -8,7 +8,7 @@ SPDX-FileCopyrightText: 2021 The CC: Tweaked Developers
 SPDX-License-Identifier: MPL-2.0
 -->
 
-The [`event!redstone`] event is fired whenever any redstone inputs on the computer change.
+The [`event!redstone`] event is fired whenever any redstone inputs on the computer or [relay][`redstone_relay`] change.
 
 ## Return values
 1. [`string`]: The event name.
@@ -21,3 +21,7 @@ while true do
   print("A redstone input has changed!")
 end
 ```
+
+## See also
+ - [The `redstone` API on computers][`module!redstone`]
+ - [The `redstone_relay` peripheral][`redstone_relay`]