Add a sparse FIR filter implementation

A Finite Impulse Response filter implementation which takes advantage of sparse coefficients.
The coefficients are assumed to be uniformly distributed and have an initial offset.

BUG=webrtc:3146
R=andrew@webrtc.org, bjornv@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49659004

Cr-Commit-Position: refs/heads/master@{#9002}
6 files changed