Extract hex_to_base36 and base36_to_hex methods in Uuid type

replacing normalize_base36. This should make it easier to convert
UUIDs between hex and base36 formats, useful when debugging production
logs that contain hex UUIDs.

For example, if in a log you see:

    User Load (0.3ms) SELECT `users`.* FROM `users` WHERE
    `users`.`account_id` = x'019afab815897a4f920f3a24fab75400' AND
    `users`.`id` = x'019afab815a2790688c58c7f8326e700' LIMIT 1

and you want to find that account or that user in the console, you can
now do:

    id = ActiveRecord::Type::Uuid.hex_to_base36("019afab815897a4f920f3a24fab75400")
    => "03f6bilrvt3oghzhurll4pp8g"

test/test_helper.rb

@@ -154,8 +154,7 @@ module FixturesTestHelper
 
       # Format as UUID string and convert to base36 (25 chars)
       uuid = "%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-%02x%02x%02x%02x%02x%02x" % bytes
-      hex = uuid.delete("-")
-      hex.to_i(16).to_s(36).rjust(25, "0")
+      ActiveRecord::Type::Uuid.hex_to_base36(uuid.delete("-"))
     end
   end
 end