expr: improved parser

[mknos]

• The old expression parser didn't work for some basic expressions I tested
• It was a bit confusing to debug the old code because everything happened in the one evaluate() function
• Replace the code with a parser based on the OpenBSD version
• Different operator priorities are delegated to different functions so overall this is a little clearer
• Add COPYRIGHT and AUTHOR sections in pod

%perl expr \( 1 \+ 1 \) \* 200
400
%perl expr "true" '&' ''
0
%perl expr 0 '|' 'true'
true
%perl expr 2 '*' 3 '+' 4
10
%perl expr 2 '+' 3 '*' 4
14
%perl expr "hotel" \: "ho"
2
%perl expr "hotel" \: "tel" # no match: regex is implicitly tied to start of string
0

[coveralls]

Pull Request Test Coverage Report for Build 12379350747

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 72.464%

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Totals
Change from base Build 12366204341:	-0.7%
Covered Lines:	350
Relevant Lines:	483

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💛 - Coveralls