Allow equipping pocket computers on the bottom

This allows equipping pocket computers on both the back (as before) and
bottom of a pocket computer. The asymmetry is a little unfortunate here,
but makes some sense with the crafting recipe (above goes behind, below
goes on the bottom).

 - Move some functionality from IPocketAccess into a PocketComputer
   interface (and PocketComputerInternal) interface, used by the pocket
   API.

   IPocketAccess preserves the same interface as before. Unlike
   ITurtleAccess, we /don't/ expose the PocketSide in the public API.

 - Several pocket-computer methods (e.g. setUpgradeData, setColour) are
   now required to be called on the main thread, and when the computer
   is being held. This allows us to write back changes to the item
   immediately, rather than the next time the item is ticked.

   Sadly this doesn't actually remove the need for onCraftedPostProcess
   as I'd originally hoped, but I think does make the code a little
   simpler.

 - Rename "computercraft:pocket_computer" component to
   "computercraft:back_pocket_computer".

 - And finally, support multiple upgrades on the pocket computer. This
   is actually quite an easy change, just tedious — there's lots of
   places to update!

Fixes #1406, and I think fixes #1148 — you can use a speaker to notify
you now.

projects/core/src/main/resources/data/computercraft/lua/rom/programs/pocket/equip.lua

@@ -7,9 +7,27 @@ if not pocket then
     return
 end
 
-local ok, err = pocket.equipBack()
-if not ok then
-    printError(err)
-else
-    print("Item equipped")
+if select('#', ...) > 1 then
+    local programName = arg[0] or fs.getName(shell.getRunningProgram())
+    print("Usage: " .. programName .. " <side>")
+    return
+end
+
+local function equip(fn)
+    local ok, err = fn()
+    if not ok then
+        printError(err)
+    else
+        print("Item equipped")
+    end
+end
+
+local side = ... or "back"
+if side == "back" then
+    equip(pocket.equipBack)
+elseif side == "bottom" then
+    equip(pocket.equipBottom)
+else
+    printError("Unknown side. Expected 'back' or 'bottom'.")
+    return
 end

projects/core/src/main/resources/data/computercraft/lua/rom/programs/pocket/unequip.lua

@@ -7,9 +7,27 @@ if not pocket then
     return
 end
 
-local ok, err = pocket.unequipBack()
-if not ok then
-    printError(err)
-else
-    print("Item unequipped")
+if select('#', ...) > 1 then
+    local programName = arg[0] or fs.getName(shell.getRunningProgram())
+    print("Usage: " .. programName .. " <side>")
+    return
+end
+
+local function unequip(fn)
+    local ok, err = fn()
+    if not ok then
+        printError(err)
+    else
+        print("Item unequipped")
+    end
+end
+
+local side = ... or "back"
+if side == "back" then
+    unequip(pocket.unequipBack)
+elseif side == "bottom" then
+    unequip(pocket.unequipBottom)
+else
+    printError("Unknown side. Expected 'back' or 'bottom'.")
+    return
 end