Readability Improvements#4

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BlakeMScurr wants to merge 3 commits intogeometryxyz:mainfrom

BlakeMScurr:main

Commits on Jun 1, 2023

Tidy up ExpandMessageXmd.
BlakeMScurr
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Commits on Jun 6, 2023

Simplify CMOV.
BlakeMScurr
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Commits on Jun 15, 2023

Refer to spec and explain non-obvious constant.
BlakeMScurr
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