Syntax-rules should freeze the scope variables when defined

[jcubic]

Found another edge case:

(define-syntax g
  (syntax-rules ()
    ((g 2) -3)))

(let-syntax ((f (syntax-rules ()
                  ((f 1) (g 2)))))
  (set! g (lambda (x) -1000))
  (f 1))

It evaluates to -1000 because syntax-rules macros are expanded at runtime, but most important is that this can be easily fixed. The macro when invoked needs to collect names from env and then use it when invoked.
This is related to long-standing task #43 but it would be easier to track as a separate issue.

Things to consider the redefinition of macro a warning. The finding is based on Scheme Servey.

[jcubic]

This may require #169 to be added. Moving to 1.1

[jcubic]

Hygienic macros cannot expand in referentially-opaque
code: if a macro expansion introduces a free identifier,
the identifier refers to the binding occurrence which
was in effect when the macro was defined rather than
when the macro was expanded.
-- How to Write Seemingly Unhygienic andReferentially Opaque Macros withSyntax-rules