Implement overdrive in resonant filter

This patch implements nonlinear distortion in the resonant ladder
filter. It's based on the differential equations in the Huovilainen '04
DAFx paper, but using matrix exponential to compute the evolution of the
state variables. The implementation is scalar (and calls into sqrtf),
but designed to be implemented in very efficient SIMD.
Neon-optimized ladder filter

Extremely highly optimized implementations of the ladder filter in Neon
assembler. The audio loops (both linear and non-linear) are hooked up
and running, while the matrix generation is still running in scalar
code, though the Neon version has been tested and benchmarked.

Performance numbers on Nexus 10: linear audio loop = 22.5 cycles.
Nonlinear audio loop = 62 cycles. Matrix generation = 580 cycles.

Note that the current code will crash on ARM v7 devices without Neon
(for example, Motorola Xoom).