This makes all remaining large stack allocations use the vararray
macros.
This continues the work of 6f2d9f50 to allow compiling with
NONTHREADSAFE_PSEUDOSTACK to move the memory for large buffers
off the stack for devices where it is very limited.
It also does this for some additional large buffers used by the
PLC in the decoder.
This should reduce computation, reduce stack usage, and be
substantially easier to read, but should not change behavior.
It's not quite bit-exact because I collapsed the application of the
decay and the fading into a single pass to save one multiply per
sample, but the difference signal is silent.
It also changes the comments into complete sentences.
We always define OPUS_BUILD, so this code has no effect.
Likewise there is no reason for it to be conditional.
According to Jean-Marc it's left over from when we had
an alternate CELT_BUILD define.
