Preserve magic constants in signatures

[jbuncle]

Summary

• preserve PHP 7 magic constants explicitly in default expressions, class constant values, and namespace constant values
• stop relying on incidental string casting for __DIR__, __FILE__, __NAMESPACE__, and __CLASS__
• add characterization coverage showing magic constant changes affect semantic version diffing

Validation

• udocker run -v /data/data/com.termux/files/home/projects/jbuncle/php-autosemver:/app -w /app composer2 sh -lc "find src bin tests -type f -name '*.php' -print0 | xargs -0 -n1 php -l && COVERAGE_MIN=90 COVERAGE_JSON=/tmp/coverage.json COVERAGE_SUMMARY=/tmp/coverage-summary.md phpdbg -qrr tests/coverage.php"
• coverage: 91.50% (904/988)