Mildly optimize RangeMap

[marc0246]

Following are the results of my benchmarks before and after:

test tests::descriptor_set    ... bench:  96,200,265 ns/iter (+/- 3,663,596)
test tests::dispatch_indirect ... bench:  86,317,442 ns/iter (+/- 1,821,521)

test tests::descriptor_set    ... bench:  67,867,435 ns/iter (+/- 1,519,747)
test tests::dispatch_indirect ... bench:  64,132,526 ns/iter (+/- 875,303)

Nothing spectacular unfortunately. I removed the Gaps iterator because I didn't have the mental capacity to re-imlement it with the new ways the ranges are stored, but I doubt we'll ever need that one anyway.

[Rua]

Still a welcome improvement, thanks!