ffi/unsafe: add `_size', `_ssize', `_ptrdiff', `_intmax', `_uintmax'

These additions could create backward-compatibility problems, but our
searches suggest that problems will be rare; it's more common for
`_size_t' to be incorrectly aliased to `_int', so having definitions
for these standard types is likely to avoid future problems.
racket/base: add `impersonator-ephemeron'
racket/snip: fix `style-list%' `notify-on-change' and contracts

The method uses the new `impersonator-ephemeron' function to
retain a callback as long any value that it impersonates
is reachable.
Revert "avoid a wrapper on callbacks that get stored in a weak table"

This reverts commit d1357f229430f6288d037791782b00b441cad4cb.

Commit 38d6afe07c fixes the problem more generally.
fix cross-reference