Describe cv modifiers and checked pointer types.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc#15

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Checked pointer types can be used with cv modifiers. Add an explanation of
this to the design. Add examples of variable declarations that use pointer
types with CV modifiers. This addresses issue #15.

This also includes a few other changes:

• Explain that the idea of span goes way back in computer science. The
  name is new, but the idea has a long history. Cite a paper on implementing
  ALGOL 60.
• Improve the description of the new checked pointer types in the introduction. This addresses some of the remaining work for issue Do not use safe and unsafe to describe pointer types. #14, which is to be clearer about what is being checked.
• Remove some awkward phrasing in the description of bounds-safe interfaces
  in the introduction.