Make many conversions sampleCount->size_t are explicit and checked...

... with run-time assertions.

I examined each place and reasoned that the narrowing was safe, and commented
why so.

Again, there are places where the sampleCount variable will later be changed
to have a different type, and they are not changed here.

src/effects/ChangeSpeed.cpp

@@ -487,6 +487,7 @@ bool EffectChangeSpeed::ProcessOne(WaveTrack * track,
 
    float * inBuffer = new float[inBufferSize];
 
+   // mFactor is at most 100-fold so this shouldn't overflow size_t
    auto outBufferSize =
       (sampleCount)((mFactor * inBufferSize) + 10);
    float * outBuffer = new float[outBufferSize];