Fix env issue

[eigenhombre]

See https://github.com/eigenhombre/smallscheme/blob/master/smallscheme/scheme.py#L242

Get more tests for closures, etc. and make sure environments and nesting of same work correctly.

[eigenhombre]

Failing test:

 ;; Lexical closures:
 (define a 3)
 (define (f) (+ 1 a))
 (is (= 4 (f)))
 (define a 4)
 ;; Fails, Issue 4:
 ;; (is (= 4 (f)))

[eigenhombre]

Scratch that - looks like I don't understand scope in Scheme:

Welcome to Racket v8.5 [cs].
>  (define a 3)
 (define (f) (+ 1 a))
 (= 4 (f))
 (define a 4)
 (= 5 (f))
> > #t
> > #t
>

[eigenhombre]

Instructive test case at https://stackoverflow.com/questions/526082/