We used to not care about stopping an audio output stream for Intel HDA
since AudioServer would continuously send new buffers to play. Since
707f5ac150ef858760eb9faa52b9ba80c50c4262 however, that has changed.
Intel HDA now uses interrupts to detect when each buffer was completed
by the device, and uses a simple heuristic to detect whether a buffer
underrun has occurred so it can stop the output stream.
This was tested on Qemu's Intel HDA (Linux x86_64) and a bare metal MSI
Starship/Matisse HD Audio Controller.
This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.
Also, move the panic and assertions handling code to that directory.
This is an implementation that tries to follow the spec as closely as
possible, and works with Qemu's Intel HDA and some bare metal HDA
controllers out there. Compiling with `INTEL_HDA_DEBUG=on` will provide
a lot of detailed information that could help us getting this to work
on more bare metal controllers as well :^)
Output format is limited to `i16` samples for now.
