Can language be added to specify that all checks should/must be disabled
if the `debug` R7RS feature identifier is not set? Currently that is the
case in the generic sample implementation because it uses SRFI 145
`assume`, but in the spec there is nothing (that I can see) to indicate
whether/how these checks can be turned off.

Also the SRFI should more clearly specify when coercion is done IMO.
Currently it says the `values-checked` form "might" coerce, but I think
the default should be no coercion (and specify this for all forms), and
that `*-coerce` variants be supplied, which must coerce when possible.

Thanks for the SRFI, I'm looking forward to using it!