reverb, loudness: memoize

[passyur]

Two optimizations to loudness in addition to changes in #103 that I'll just copy the text of here:

for each note, every partial was being expanded into numChannels full-length buffers during spatialize_Track. for the default spatializer, each of those buffers were identical copies scaled by 1/numChannels. this change leverages the fact that these buffers are copies of an initial buffer by performing the per-partial spatialization computation outside of the per-sound loop (and during the unification step).

These are pretty lucrative changes, as they remove a bunch of redundant allocs that ran in the Loudness::calculate loop by memoizing RELATIVE_AMPLITUDE per partial and calculating LOUDNESS once (as it's constant). Also removes a pointless copy-and-replace of the percentReverbinput argument. Finally, Envelope::getValue now has a fast path for linear segments.