This has mainly performance benefits, so that we only need to call into
all processors once for every audio buffer segment. It requires
adjusting quite some logic in most processors and in Track, as we have
to consider a larger collection of notes and samples at each step.
There's some cautionary TODOs in the currently unused LibDSP tracks
because they don't do things properly yet.
Previously, a collection of notes (Vector or Array) would be created and
promptly deleted for every sample (at least 44 thousand times per
second!). This was measured to be one of the most significant
performance drawbacks as well as the most obvious performance
improvement I could currently find here. Although it will not cause
Piano to lag currently (at least on virtualized systems), I see an
incoming issue once we get the capability to use more processors.
Now, we use a HashMap correlating pitches to notes, and Track reuses the
data structure in order to avoid reallocations. That is the reason for
introducing the fast clear_with_capacity to HashMap.
