Improve integer types.

This removes the PCM[N] data types, and renames the PCM[N]Samples data
types as follows:

PCM8         -> uint8
PCM16        -> int16
PCM32        -> int32
PCM8Samples  -> type PCM8 []uint8
PCM16Samples -> type PCM16 []int16
PCM32Samples -> type PCM32 []int32

All PCM[N]ToFoo and FooToPCM[N] functions have been renamed as well.