library: use strict provenance lints consistently

The `fuzzy_provenance_casts` lint is enabled in most of the standard
library, but its identical twin `lossy_provenance_casts` was not. As
discussed in the tracking issue for those lints, there doesn't seem to
be any good reason to enable one without the other. This PR applies this
principle and as a result removes some unnecessary ptr->int `as` casts.

It's also preparation for merging the two lints, which removes the
option of only enabling `fuzzy_provenance_casts`.