Also fixes some indenting in opusfile_example.

This is pretty preliminary for now.
Seeking performance isn't great, and there's no caching.
I'm sure there's also lots of bugs.

When a match fails, the index of the position in the link needs to
 be 64 bits.

1) Check for allocation failure and fall back to merely scanning a
 file for consistent timestamps instead of loading it to RAM.
2) Report holes, but do not quit scanning/loading unless they cause
 a discontinuity in the timestamps.

This matches the convention used by opus itself, where
headers are included without a prefix, and the build
must do -I/path/to/opus to find the appropriate versions.

Move this out of opusfile_example and into the API proper.

Seeking and playback work, at least as far as the examples test
 them.
There are probably still lots of bugs.