Add a new FLACNavigator class that can scan for frames both forward and
backward from a byte offset to get the exact buffered ranges. The reads
are done in chunks, which keeps each call pretty cheap.
The ranges themselves are resolved by its ScanningContainerNavigator
base class, which reconciles cached ranges against the incoming ones to
avoid repeating work unnecessarily. With those combined optimizations,
the calls normally take under 1 microsecond.
