bug#24102: Use guile variable objects as SRFI-111 boxes.

["Glenn Michaels" <gmichaels@Safe-mail.net>]

Currently, guile's (srfi srfi-111) module ("mutable boxes") provides
an implementation based on records with a single value field.

Wouldn't it make more sense to re-export the functions make-variable,
variable?, variable-ref and variable-set! from the guile core as box,
box?, unbox and set-box! respectively?

These functions have the same signatures and the same semantics as
required by the SRFI-111 spec., and they appear to be significantly
faster than the current record-based implementation.

Moreover, SRFI-111 boxes and guile variable objects are clearly
semantically the same thing. It's bad enough having two names for the
same thing, without having two implementations too.

Reference: http://srfi.schemers.org/srfi-111/srfi-111.html