Convert C pointers to Go pointers

[dominikh]

Instead of wrapping C pointers in Go structs, wrap C values in Go
structs, then convert C pointers to Go pointers. This lets us use
actual pointers in the API, simplifying things like nil pointer
checks.

We use the notinheap hint for some added safety around C pointer
rules (i.e. users can't accidentally allocate their own wrappers on
the Go heap and pass them to C) and for some added performance (as the
compiler can avoid generating barriers and whatnot when accessing
these pointers.)

Note that this still won't let us set finalizers, as the pointers
aren't managed by Go and thus don't get garbage collected.

[diamondburned]

I think this PR conflicts with #2. If we pass around C pointers (as Go
pointers), then we cannot attach finalizers on them.