Sorted out the tap/send/insert naming confusion

The terminology around the 'xinsert' unit and related APIs
somehow ended up pretty much the inverse of what one would
commonly find in studio mixing consoles and the like. Now
it's been "corrected" as such;

a2_TapCallback() has been renamed as a2_SendCallback().
a2_SendCallback() has been renamed as a2_ReturnCallback().
(a2_InsertCallback() remains unchanged.)

a2_OpenTap() has been renamed as a2_OpenSend().
a2_OpenSend() has been renamed as a2_OpenReturn().

Hopefully, this is less confusing to people familiar with
studio mixers and software using traditional terminology.

Closes #138.