Improve audio interleaving

[yudori]

I found a pretty difficult-to-reproduce scenario where the interleaving code would get 'stuck' following line

crunker/src/crunker.ts

Line 407 in 0ac03d4

result[index++] = input.getChannelData(channelIdx)[inputIndex];

where we're repeatedly calling input.getChannelData() potentially for millions of data in each audio channel.

I made the change to make this call only N times where N = number of channels and store the channel data in an array instead.

This PR also includes an early return in the interleaving code for scenarios where there's a single channel buffer which saves some memory and improves performance.

[jaggad]

I have some very minor stylistic nitpick preferences, but the change to the logic itself looks very good so I won't get bogged into that. Clear performance win.