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.
