Hello all,

This patch implements a new primitive syntax form 'with-ellipsis' that
changes the ellipsis identifier within a lexical scope, and uses this to
implement the R7RS extension to 'syntax-rules'.  I'd like to apply this
to the stable-2.0 branch.

It took me quite a while to figure out the right way to generalize this
R7RS feature for the syntax-case system.  My first idea was to make
'syntax' a syntax parameter, but further thought made it clear that this
would be inadequate.  Consider all of the other macros based on 'syntax'
and 'syntax-case': things such as 'with-syntax', 'quasisyntax', etc.

It was obvious that 'with-ellipsis' should modify the lexical
environment somehow, but how?  First of all, consider that a single
'syntax' or 'syntax-case' form might, in general, be composed of
identifiers from many different lexical environments.  The lexical
environment of the 'syntax' keyword is not the right environment to look
at for the entire template, because in a 'quasisyntax' or 'syntax-rules'
form, that's the lexical environment of the 'quasisyntax' or
'syntax-rules' macro.

It soon became clear that I must search the lexical environment of each
identifier within a pattern or template to determine if that identifier
is an ellipsis.  But what identifier should it look up in that lexical
environment?  Again, my first thought was to look up the identifier
itself, but that's not right either.  A 'with-ellipsis' form must not
only add a new ellipsis identifier; it must also disable the special
meaning of the previous ellipsis identifier.

In the end, here's how this works: 'with-ellipsis' binds a special
identifier named #{ $sc-ellipsis }# using a new 'ellipsis' binding type.
The new ellipsis identifier is stored within the binding.  In order to
determine whether an identifier X is an ellipsis, the binding for
#{ $sc-ellipsis }# is looked up in the lexical environment of X.  If the
binding is found and has binding-type 'ellipsis', then X is compared to
the identifier stored in the binding using 'bound-id=?'.  Otherwise, X
is compared to '...' using 'free-id=?' as was done before.

After much thought, I'm fairly confident that this is the only good
approach.  I'd like to push this to stable-2.0.

Comments and suggestions welcome.