Store "marked" flag in high bit of "roots" counter

[mwerezak]

Should reduce memory footprint. I ran the benchmarks with cargo +nightly bench and didn't notice any slowdown, at least on my arch (x86_64-pc-windows-msvc).

Before:

test discard_100   ... bench:      12,092 ns/iter (+/- 612)
test discard_10000 ... bench:   1,197,697 ns/iter (+/- 26,895)
test keep_100      ... bench:      10,594 ns/iter (+/- 284)
test keep_10000    ... bench:     975,823 ns/iter (+/- 42,083)

After:

test discard_100   ... bench:      11,111 ns/iter (+/- 217)
test discard_10000 ... bench:   1,111,062 ns/iter (+/- 35,206)
test keep_100      ... bench:      10,945 ns/iter (+/- 773)
test keep_10000    ... bench:   1,052,570 ns/iter (+/- 53,772)

Incrementing and decrementing roots requires some more bit operations but hopefully those are pretty cheap? Also instead of checked_add() I use a branch, I don't fully know what the performance implications of that would be. I imagine not much because its probably a very predictable branch.

[mwerezak]

I can remove the #[inline]s if you think they are unnecessary.