Performance optimizations

[colinodell]

Alternative PR to #388 with many of the same optimizations. A huge thanks to @ricardofiorani for implementing most of these! 🎉

Before:

Running Benchmarks (Isolated), 7 Implementations, 20 Iterations:
	CommonMark                     ....................
	PHP Markdown                   ....................
	PHP Markdown Extra             ....................
	Parsedown                      ....................
	cebe/markdown                  ....................
	cebe/markdown gfm              ....................
	cebe/markdown extra            ....................

Benchmark Results, CPU:
	1) Parsedown                    1.67 ms              
	2) cebe/markdown                1.74 ms      -0.07 ms
	3) cebe/markdown extra          2.01 ms      -0.34 ms
	4) cebe/markdown gfm            2.45 ms      -0.78 ms
	5) PHP Markdown                 3.09 ms      -1.42 ms
	6) PHP Markdown Extra           4.25 ms      -2.58 ms
	7) CommonMark                  20.65 ms     -18.98 ms

Benchmark Results, Peak Memory:
	1) PHP Markdown                 2662 kB              
	2) Parsedown                    2806 kB       -144 kB
	3) PHP Markdown Extra           2898 kB       -236 kB
	4) cebe/markdown                2912 kB       -250 kB
	5) cebe/markdown extra          2982 kB       -320 kB
	6) cebe/markdown gfm            3337 kB       -675 kB
	7) CommonMark                   6761 kB      -4099 kB

After:

Running Benchmarks (Isolated), 7 Implementations, 20 Iterations:
	CommonMark                     ....................
	PHP Markdown                   ....................
	PHP Markdown Extra             ....................
	Parsedown                      ....................
	cebe/markdown                  ....................
	cebe/markdown gfm              ....................
	cebe/markdown extra            ....................

Benchmark Results, CPU:
	1) Parsedown                    1.58 ms              
	2) cebe/markdown                1.71 ms      -0.13 ms
	3) cebe/markdown extra          2.00 ms      -0.42 ms
	4) cebe/markdown gfm            2.36 ms      -0.78 ms
	5) PHP Markdown                 2.92 ms      -1.34 ms
	6) PHP Markdown Extra           4.28 ms      -2.70 ms
	7) CommonMark                  17.93 ms     -16.35 ms

Benchmark Results, Peak Memory:
	1) PHP Markdown                 2666 kB              
	2) Parsedown                    2810 kB       -144 kB
	3) PHP Markdown Extra           2902 kB       -236 kB
	4) cebe/markdown                2916 kB       -250 kB
	5) cebe/markdown extra          2986 kB       -320 kB
	6) cebe/markdown gfm            3341 kB       -675 kB
	7) CommonMark                   6734 kB      -4068 kB