Make cx_math_is_zero run in constant time

[bigspider]

Description

The current implementation cx_math_is_zero does not run in constant time, but it's not documented; this could lead to vulnerabilities if used in cryptographic code in apps.

This PR replaces it with a constant time implementation to avoid any such risks.

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.49%. Comparing base (98eaa57) to head (fa3f9b8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #823   +/-   ##
=======================================
  Coverage   35.49%   35.49%           
=======================================
  Files          13       13           
  Lines        1606     1606           
=======================================
  Hits          570      570           
  Misses       1036     1036           

Flag	Coverage Δ
unittests	35.49% <ø> (ø)

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