[plt-scheme] immutable vector and box literals

As of 3.99.0.10, vector and box literals are immutable:

 > (vector-set! '#(1 2 3) 0 0)
 vector-set!: expects type <mutable vector> as 1st argument....

Along the same lines, `syntax->datum' produces immutable vectors and
boxes instead of mutable ones.

I doubt that this change will be controversial, but it may be
interesting to check whether anyone is using mutable literals as a back
door to share across instantiations of a module.

Matthew