Add signed/fixed-point width macros

[darsor]

This PR adds macro definitions for the intwidth (_iw), fracwidth (_fw), and is_signed (_signed) UDPs per https://peakrdl-regblock.readthedocs.io/en/latest/udps/fixedpoint.html.

Docs and unit tests have been updated.

[coveralls]

Pull Request Test Coverage Report for Build 15352420070

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 94.878%

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Files with Coverage Reduction	New Missed Lines	%
/home/runner/work/PeakRDL-cheader/PeakRDL-cheader/src/peakrdl_cheader/header_generator.py	2	97.82%

Totals
Change from base Build 14720272370:	-0.5%
Covered Lines:	556
Relevant Lines:	575

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💛 - Coveralls