Collection of various refactoring and fixes.

[pointybeard]

This includes a number of changes from myself as well as a PR#1 from vbartusevicius:

• fix: Update Lexer to extend AbstractLexer
• chore: Add .phpunit.result.cache to .gitignore
• chore: Update composer library versions and use psr-4 autoloader
• chore: Add PHP 7.4 and 8.0 to .travis.yml
• fix: Incorrect namespace for Doctrine Lexer base class
• refactor: Update tests to use assertMatchesRegularExpression() instead of deprecated assertRegExp()
• refactor: Remove use of abandoned patchwork/utf8 library.
• fix: Propagate the call to mt_rand() by assigning it to $seed (Fixes SimpleRandom constructor does not set seed value randomly if $seed is null #6)
• use PHP_INT_MAX for largest possible max instead of hardcoded value