Finish replacing MODULE/module with SPACE/space
Replace 'shortcut object' with 'space function' in tests
Clarify documentation: space() returns the value of the callback

I also fixed the code sample for the equivalent code in WITHIN1 section,
which used space() where within() was intended.
Return a space function instead of a space object

The next step is to add unit tests for the new behavior,
and to move the behavior on the last line of within() to the
implementation of the function, replaced with:

  return space( callback );
Complete unit tests for space object -> space function
Fix unit tests

I discovered the errors by running the tests on the complete implementation,
which now passes all tests (next commit).
Complete implementation of space function

I applied the expected refactoring, moving and replacing the
implementation with a call to the new function:

  return space( callback );
Only add methods to the space function when it is returned

These 4 assignments can be skipped when the space function
is run directly instead.
Add inline comments to clarify obscure points of the implementation