consider `begin` variant with toplevel semantics that can be used locally

[felix winkelmann]

(##core#toplevel-begin BODY ...): like begin, but do not canonicalize the BODY.

This allows wrapping toplevel code with a procedure, for example:

(define (toplevel)
  (##core#toplevel-begin
    (define a 1)
    ...))

...

(toplevel)       ; initialize
a

[felix winkelmann]

aded in 4722762d (experimental).