Makes the SILK pitch search use celt_pitch_xcorr()

Should gives us ARM/SSE optimizations for free.
Share auto-correlation code between SILK and CELT
Converts the fixed-point silk_burg_modified() to use celt_pitch_xcorr()

bit-exact with original version
Implements fixed-point silk_LPC_analysis_filter() in terms of celt_fir()

Saves 2.5% on ARM without any asm. The float build still uses the old
code because celt_fir() then becomes a float function.
Surround masking rewrite

The channel energy is now computed using an MDCT in the multi-stream
encoder rather than computing the energy of the stereo downmix.
Makes surround_analysis() work in fixed-point
Adds trim and dynalloc control to surround masking
Moves the analysis back to opus_encode_native() to it runs for all streams.

I checked that it's not *completely* busted, but it probably needs more testing.
First pass at making the analysis code run with FIXED_POINT

Code is still float, but at least tonality esitmation seems to work.
Speech/music analysis is still disabled.
Makes speech/music detection work with FIXED_POINT (code still float)
Analysis scaling fixes

This should make the scaling the same for fixed and float. It changes the float
scaling too by normalizing by the number of channels, which matters for
bandwidth detection.
Fixes delayed decision for fixed-point
Adds surround masking to SILK
Code for handling upsampling in surround_analysis()

Still untested.
Surround analysis should now work for non-20ms frame sizes.

Not well tested.
Tuning the surround masking

We're now a bit more conservative and only take into account the bands
that are being codec. Also weighting the rate saving by the band width.
More surround masking tuning
Allowing surround to use lower bandwidth than fullband
Very basic surround rate calibration
Moves frame size selection back out of opus_encode_native()

Should fix delayed decision on surround, but not well tested.