Add "scopes" chapter.

[ehuss]

This adds the chapter on scopes.

I tried to make this accurate and complete, but it is probably neither. There are some things that are intentionally not covered, such as some ambiguities (will handle in other places or at a later date), peculiarities with macros (perhaps deferred for more explicit "expansion" docs?), details regarding use (will add to use chapter), and known bugs (hopefully they will get fixed some day).

cc #568

[ehuss]

@petrochenkov Is this something you could potentially review/look over?

[ehuss]

Thanks for the reviews!