Reorganize DynamicCasting spec

This only rearranges the existing sections; it does not change any technical points.

This attempts to make the spec a little clearer by grouping the individual type discussions under four main headings:

* Classes and Foreign Types.  This includes Swift classes, Obj-C classes, and CF types.
* Other Concrete Types.  Struct, Enum, Tuple, Function, Optional, and Set/Dict/Array
* Existential Types.  Any, AnyObject, Error, Protocol types, and AnyHashable(*)
* Metatypes & Existential Metatypes (*)

This organization seems to flow a little better.

In particular, it gives me a place to discuss issues common to all Existential types, which I'll work through in a subsequent PR.

Footnotes: This organization isn't perfect, of course:
 * AnyHashable isn't really an Existential type, but it behaves as such for casting purposes, so it makes the most sense to discuss it in the Existentials section.
 * Metatypes are technically concrete types, but it seems to make more sense to discuss them after discussing Existential types.

Author: tbkka