Extracting High and Low 32bit words from 64bit value

[phanikishoreg]

Add macros and clean-up code to extract High and Low 32bit words from 64bit value. Perhaps also to extract higher_n or lower_n bits! Which might be useful in extracting 16bit from 32bit values, and other more common bit manipulations as macros.

[gparmer]

Yes, making this generic is important for when we port to different platforms. See the round_*_to_* functions for an example of this type of generality.

[gparmer]

Likely in consts.h along with the other pow-2 math.