[doc,library] Document parser expansions as an Open Dylan extension.

This extension was originally created at Harlequin by Andrew Armstrong,
Keith Playford and Jonathan Bachrach.
[doc,library] Document macro system extensions.

As indicated in the text, the bulk of this text is from the
D-expressions paper.
Merge pull request #733 from waywardmonkeys/macro-extensions-docs

[doc,library] Document macro system extensions.
Merge pull request #734 from cgay/docs

[doc] Fix formatting, typos, bugs in compiler-internals.rst.
Merge pull request #735 from cgay/docs

[doc] grammar fixes for compiler-internals.rst
[dfmc] Fix grammar in a comment.
[doc] Fix typo and whitespace in macro-system-extensions.rst