On a 64-bit machine, the slices in a Container consume 72
bytes, and the Container itself is 80. But we only use one
slice at a time! This patch shifts us to keeping a single
slice in the Container, and converting provided slices to
and from that type when we want to update it. (It is not
safe to access the slice through the wrong type.)
We also add some new tests, conditional on a build tag
called `roaringparanoia`. These tests will be optimized
away entirely by the compiler when the tag isn't
present, because the conditionals use a const. These catch
possible errors like trying to access the bitmap slice
of a non-bitmap container.
We also eliminate all direct creation of Container literals,
so we can mess with the internals more. (On reflection
and study, we decided not to go to the fancier design where
references to .n and .typ were also converted to function
calls, which would have allowed packing those attributes
more tightly, because it was a lot more overhead and a lot
of work to keep track of.)
There's some circumstances where we appear to have been
relying on incorrect guesses about the nature of containers.
For instance, in xorBitmapRun, there's logic that makes sense
only if the output's a run container, but it's not, it's a
bitmap container. This creates strange behavior sometimes,
though. Several of these are corrected now.
