[racket] Expand string for use with enter!

The `enter!` form is analogous to `require`, where both are syntactic
forms. It sounds like you want something analogous to
`dynamic-require`, but there's currently no `dynamic-enter!`.

We can add `dynamic-enter!` to `racket/enter`. Meanwhile, though, you
may have to use `eval` as a workaround.

At Mon, 6 Jan 2014 18:07:35 -0600, Mark Watts wrote:
> Hello,
> 
> I'm trying to make a wrapper for enter! (from racket/enter) but the
> filename argument gets read by enter! as a symbol rather than the value it
> is bound to. How can I expand the argument into a string for this case?
> 
> -- 
> 
> Cheers,
> Mark Watts
> Department of Computer Science
> University of Texas at Austin
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