Document scheme parser extensions

[haase]

Including:
choices {} and qchoices #{}
compounds #%
packets and secrets
@ (oids), including @? and others
Other # macros, and #:name: macros

[haase]

Include explanation of quasiquote, and the #, syntax for templates.

Also describe history references, referring to a spearate document as needed.

[haase]

First pass done.