requiring itself into the entered namespace.
This makes it useful in some cases where this require leads to a
dependency cycle, eg (enter! racket/list). It's obviously not useful
for use as-is, since you will not have a bound `enter!' to get out of
the namespace (and possibly no `require' to get it) -- but it is useful
for meta-tools like xrepl. This is why the flag is verbose. `xrepl'
now uses this flag.
Also, the check for valid keywords for the form is now done at runtime
rather than in the macro. This doesn't matter in this case, since the
form is intended for interactive use anyway.
Also, separate the two parts of `enter-load/use-compiled' (it was
defined curried, but didn't use it).
