Add .size convenience method;

This is now used as the default in the Buffer class.
Add simple value class, also;

- require_relative supporting libraries.
Buffer shall accept Strings and Arrays;

- Remove #content method (not the job of Buffer).
- Remove #invalid_cell checking method.
Add Mask class for Colour pairs.
Remove skipped test.
Add test for version, y'know for completeness.
Add simple Renderer class.
Handle Buffers in Compositor.
Add 'bold' and 'underline' escape codes;

Also, reorder methods.
Yard: Update method documentation.
Rename 'pair' to 'mask'.
Rename 'interpolate' to 'compose'.
Rename 'buffer' to 'composition'.