Writing to a mutable var is a particularly potent source of leaks since
it mostly defeats GHC's analysis. Here we add assertions to all mutable
writes, and fix a couple spots where we wrote some thunks to a mutable
var (compiled with -O2).
Some of these thunks were probably benign, but others looked liked they
might be retaining big args. Didn't do much analysis, just fixed.
Actually pretty happy with how easy this was to use and as a diagnostic,
once I sorted out some issues. We should consider using it elsewhere,
and maybe extending so that we can use it with tests, enable when
`-fenable-assertsions` etc.
Relates #3388
Also simplified codepaths that use `AcceptWith`, which has unnecessary
`Maybe` fields.
