Utilize ArrayPool for ECC calculations

[Shane32]

Improves performance by calculating generatorPolynom only a single time and renting arrays from ArrayPool. Basically I rewrote Polynom to function similar to List<PolynomItem> with methods such as Add, Clear and Sort. It's a struct so it does not require a heap allocation itself (no change), and the underlying buffer is rented from the array pool. Calling Dispose on the Polynom will return the array back to the array pool.

Before

Method	Mean	Error	StdDev	Gen0	Allocated
CreateQRCode	107.2 us	0.66 us	0.62 us	1.2207	23.18 KB
CreateQRCodeLong	1,878.4 us	4.68 us	4.15 us	23.4375	433.53 KB

After - via ArrayPool.Shared

Method	Mean	Error	StdDev	Gen0	Allocated
CreateQRCode	99.44 us	0.506 us	0.448 us	0.7324	14.68 KB
CreateQRCodeLong	1,637.04 us	2.811 us	2.347 us	1.9531	45.75 KB

After - via custom array pool implementation (for use with .NET Framework)

Method	Mean	Error	StdDev	Gen0	Allocated
CreateQRCode	98.51 us	0.188 us	0.157 us	0.7324	14.68 KB
CreateQRCodeLong	1,706.09 us	2.382 us	2.112 us	1.9531	45.75 KB

[Shane32]

I don't know why these tests pass. CalculateECCWords mutates generatorPolynom. I have to rework this PR.

EDIT: fixed now; generator polynomial is duplicated so the original doesn't change

[Shane32]

Just for fun, I've added another benchmark for a 2600-character QR code and compared this PR against v1.5.1. For these unreasonably-long QR codes, memory requirements have been reduced from 110 MB to 114 KB, a 99.9% reduction. And there's still room for improvement.

Version 1.5.1

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
CreateQRCode	245.4 us	2.36 us	1.97 us	17.0898	-	317.99 KB
CreateQRCodeLong	7,492.1 us	30.09 us	26.67 us	265.6250	15.6250	4920.13 KB
CreateQRCodeLongest	88,024.3 us	209.67 us	185.86 us	5833.3333	833.3333	110090.17 KB

This PR

Method	Mean	Error	StdDev	Gen0	Allocated	% RAM reduction
CreateQRCode	98.12 us	0.213 us	0.189 us	0.7324	14.68 KB	95%
CreateQRCodeLong	1,675.42 us	1.812 us	1.415 us	1.9531	45.75 KB	99%
CreateQRCodeLongest	14,824.82 us	48.627 us	45.486 us	-	113.96 KB	99.9%

[Shane32]

Thanks @gfoidl ! I'll merge in your suggestions here later tonight.