improve adaptive LPC order search

[justinruggles]

The speed of the adaptive LPC order search can likely be improved a good bit.
Things to try:

• Split search with direction threshold, similar to BGMC param search. Possibly only if order is above a certain value.
• Progressive autocorrelation + levinson-durbin (probably won't work with split search though)

[justinruggles]

Still working on this, but I want to push more LPC changes upstream before complicating the code even more.