Implement metamethod __eq for rectangles

This is part of #11.
Implement metamethod __tostring for rectangles

This is part of #11.
Default rectangle components to 0
Remove deprecated rectangle:unpack() function
Implement rectangle inflation

This is part of #11.
Implement rectangle intersection

This is part of #11.
Implement rectangle offset

This is part of #11.
Implement rectangle union

This is part of #11.