Change FastTree BinFinder to use floats and remove one data copy #1648

See the conversation here: #1580 (comment)

With the above change, I made it so the FastTree BinFinder.FindDistinctCounts was no longer destructive of the values VBuffer during CalculateBins.

Now that it no longer destroys the buffer, we no longer need to copy it here:

However, I couldn't easily remove this copy because doing the copy also changed the VBuffer from float to double. This should also be changed, as recognized by this REVIEW comment in the code:

This issue is to fix both of these things. First, change BinFinder to work on float instead of double. Then, we can remove this extra copy and just pass in the normal VBuffer<float> to BinFinder, without worrying if it will destroy the buffer.

/cc @Zruty0 @TomFinley