Add `:is_new/old()` helpers to tarantool.compat options

compat.<option_name>.current can be 'new', 'old' or 'default',
thus when it is default there must be an additional check if
compat.<option_name>.default is 'new'. It is handier to have a
helper to deal with that instead of complicated if:

• check if effective value is 'new' before the patch:

  if compat.<option_name>.current == 'new' or
          (compat.<option_name>.current == 'default' and
           compat.<option_name>.default == 'new') then
      ...
  end
  

• after the patch:

  if compat.<option_name>:is_new() then
      ...
  end
  

Please update tutorial on using compat, maybe add an example to
Listing options details.

Requested by @Lord-KA in https://github.com/tarantool/tarantool/commit/5f6d367c51f7545a940119e7a393baf982b80f3f.