WIP: SIMD Leftover Data

[Turnerj]

Simplifies how leftover data is processed. In some basic testing, it seems to perform better for small text (likely due to less code size needing to load up), about the same for 128-bit processing at each above that but slightly worse for 256-bit processing.

Before

Method	Job	EnvironmentVariables	RowSize	Mean	Error	StdDev	Op/s	Code Size	Allocated
Fill	Core (All Intrinsics)	Empty	10	4.523 ns	0.1255 ns	0.1446 ns	221,068,464.6	422 B	-
Fill	Core (w/o AVX2)	COMPlus_EnableAVX2=0	10	4.754 ns	0.1291 ns	0.1435 ns	210,360,789.7	385 B	-
Fill	Core (All Intrinsics)	Empty	300	20.146 ns	0.4255 ns	0.4900 ns	49,637,004.7	422 B	-
Fill	Core (w/o AVX2)	COMPlus_EnableAVX2=0	300	49.058 ns	0.9581 ns	0.8962 ns	20,384,240.0	385 B	-
Fill	Core (All Intrinsics)	Empty	8102	497.672 ns	9.9860 ns	13.9990 ns	2,009,355.1	422 B	-
Fill	Core (w/o AVX2)	COMPlus_EnableAVX2=0	8102	830.389 ns	15.7661 ns	16.1907 ns	1,204,255.5	385 B	-

After

To supply later