Add benchmark suite and profile-guided optimizations

[texodus]

This PR adds a benchmark suite which measures scroll performance, and some modest performance improvements to the library itself (guided by said benchmarks).

• Modified CSS stylesheet to use setProperty to update fields on the CSSRule rather than regeenerating the entire sheet.
• Check if CSS has changed before applying.
• Removed reduce/map/etc calls where we can save an allocation.
• Modified some code to avoid iterating children where possible (to avoid the HTMLCollection overhead).

Before:

Test	Throughput (fps)	Avg Frame (ms)	CPU %	Layout (ms)	Layout #	Style (ms)	Style #
Scroll Right	37.43	26.71	100.0	2,948.63	930	585.73	1,130
Draw	271.38	3.68	94.0	101.89	200	129.72	200

After:

Test	Throughput (fps)	Avg Frame (ms)	CPU %	Layout (ms)	Layout #	Style (ms)	Style #
Scroll Right	43.50	22.98	100.1	2,433.02	835	660.50	855
Draw	352.48	2.84	94.9	82.56	200	111.35	200