validate-syntax => validate-context-syntax

Since I'll be adding more macros that need syntax validation, I made it
more clear which syntax this code validates.
Added shared contexts

The `shared-context` macro first validates its syntax, and then
compiles the code into an env. To use the macro, use `(include-context
"NAME")` inside of a context or other shared context.
Removed the shared-context bug; improved error messages

Shared contexts were originally modifying the parent context's
description, which was unacceptable. For the error messages, misleading
information was being printed out regarding allow shared-context child
forms (see line 35 of context.lisp).
Merge pull request #5 from nahiluhmot/add-shared-contexts

Add shared contexts