Consider adding defer/scope guards

[ruricolist]

Consider adding defer (as in Go) or scope guards (as in D) as a mechanism of resource cleanup.

WIP at https://github.com/ruricolist/serapeum/blob/defer/defer.lisp.

The semantics are fairly straightforward (at least lexically; would there be a use for dynamic scope guards?). Less clear is how useful it would be.

[ruricolist]

If we make "scopes" dynamic and first-class, so we can pass them as parameters, we can implement the full generality of RAII.

This could be very useful for writing FFI code (to avoid "writing C in Lisp").