Support the direction opcode

[jpcima]

Implement #179

This supports direction=reverse.

• Instead of being identified by filename, samples are identified by filename + reverse bit.
  The class FileId is created for this purpose.
• This change is propagated throughout sfizz in places where applicable.
• For non-oscillator regions, the file will be requested in the pool with option reverse=1.
  At load time, the sound file seeks at end-N frames, reads N frames to the buffer, and reverses the result.

Note: loop is ignored on reverse=1, a comment is written to indicate it.

Example

<region>
pitch_keycenter=69
sample=mysound.wav
direction=reverse