stringToEnum: Sort enum fields before inline for

[squeek502]

As discovered in Vexu/arocc#524 (Vexu/arocc#524 (comment)), stringToEnum could have considerably better codegen for large enums if it were sorted by field length before the inline for:

zig/lib/std/meta.zig

Lines 41 to 46 in a126afa

	inline for (@typeInfo(T).Enum.fields) |enumField| {
	if (mem.eql(u8, str, enumField.name)) {
	return @field(T, enumField.name);
	}
	}
	return null;

Here's a benchmark focusing on just different possible sorting of enum fields (this is with 3948 fields in the enum, shortest field length is 3 and longest is 43):

-------------------- unsorted ---------------------
            always found: 3718ns per lookup
not found (random bytes): 6638ns per lookup
 not found (1 char diff): 3819ns per lookup

----------- sorted by length (desc) ---------------
            always found: 1176ns per lookup
not found (random bytes): 68ns per lookup
 not found (1 char diff): 1173ns per lookup

----------- sorted by length (asc) ----------------
            always found: 1054ns per lookup
not found (random bytes): 67ns per lookup
 not found (1 char diff): 1053ns per lookup

-------- sorted lexicographically (asc) -----------
            always found: 2764ns per lookup
not found (random bytes): 4615ns per lookup
 not found (1 char diff): 2750ns per lookup

This would ultimately be a trade-off between compile time and runtime performance. I haven't tested to see how much of an impact on the compile time the comptime sorting of the fields would incur. We might end up hitting #4055, in which case this optimization might need to wait a bit.

Note: Sorting would also make it easy to create a fast path that checks that the str.len is within the bounds of the longest/shortest enum field.

cc @Validark

[squeek502]

Duplicate of #3863, moved this there instead