Improve Enum.ToString perf for [Flags] enums

[stephentoub]

Two main changes:

• Rather than using a StringBuilder to insert the strings for all of the constituent values, we track the constituent values in a span, summing the expected length, and then new up a string and copy the results directly into it.
• When there's a single value that matches the supplied value, we just return the cached string rather than allocating a new one.

Method	Value	Before Time (ns)	After Time (ns)	Before Alloc (b)	After Alloc (b)	Time Improvement	Alloc Improvement
FlagsToString	2147483647	422.54	103.70	96	96	4.07x	1.00x
FlagsToString	Eighth	85.88	28.54	64	24	3.01x	2.67x
FlagsToString	Eleventh, Twelth, Sixth, Seventh, Eighth	385.98	83.00	128	128	4.65x	1.00x
FlagsToString	Fifth, Seventh	139.75	58.62	80	80	2.38x	1.00x
FlagsToString	First	85.57	34.03	56	24	2.51x	2.33x
FlagsToString	None	53.49	26.92	24	24	1.99x	1.00x
FlagsToString	Twelth	90.19	29.13	64	24	3.10x	2.67x

cc: @jkotas, @ahsonkhan, @GrabYourPitchforks, @joperezr

[jkotas]

Nice

[ahsonkhan]

nit: This is essentially iteration over all the foundItems, correct? Would writing it as a regular for loop increase readability?

for (int i = foundItemsCount - 1; i >= 0; i--)
{
   ...

   int index = foundItems[i];
   name = names[index];

   ...
}