I cleaned up all String find-like functions. They were very inconsistent implementation wise, unnecessarily long, and had one or two opportunities for optimization.

Changes

• Abstracted ranged string compare into string_equal_lower, to make implementations simpler.
• Made variable names consistent.
• Shortened implementation where possible.
• Moved bounds checks out of the loop (-> 10% speed up).
• Used memcmp where possible (-> 2x speed up).
• I missed that one of the two implementations had if (src_len == 1) already. Due to my check added in Optimize string single char contains calls. #100024, this branch is no longer necessary.
• Added a few edge case tests.

I'm pretty sure the new code doesn't change any behavior, except print no errors if p_from was too large on find.

Benchmark

My changes can improve string-string case sensitive finds (find, rfind, findmk) by 2x:

	String s1 = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.";
	String s2 = "Ut enim ad minim veniam";

	{
		auto t0 = std::chrono::high_resolution_clock::now();
		for (int i = 0; i < 10000000; i ++) {
			// Test
			int r = s1.find(s2);
		}
		auto t1 = std::chrono::high_resolution_clock::now();
		std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(t1 - t0).count() << "ms\n";
	}

• 2131ms on master (5b52b4b)
• 1054ms on this PR (3e000ed43dd6905f77c4840d1ab49bc443ab6cb7)

All other find functions will experience a ~10% speed up (tested earlier).