PEG parser-generator for PHP

This is a continuation of combyna/php-peg, which itself was "a minimally invasive fork" of the original hafriedlander/php-peg.

Main features of this fork:

Fix, Optimization: Packrat parser overhaul. Simplified logic using arrays instead of a string. Arrays ultimately seemed to be more fit for the job, memory-wise. This also fixed occasional problem with accessing undefined indexes in packrat cache.
Fixed: Catastrofic backtracking problem (sometimes happening when compiling a larger grammar) avoided by simplifying regex that searches for grammar definitions.
Fixed: Fixed tests for PHPUnit 6.5 (which is now also added as dev dependency to composer.json)
Optimization: Using native PHP constants and functions with absolute namespace is slightly faster (changed in generated code, too).
Optimization: Use strict comparisons where possible (even in generated code).