Improve performance of hashing and reduce memory

[vcsjones]

👋 I had a look at the use of hashing here for detecting duplicate images. Currently, this will allocate an array of bytes per pixel-row of each image, which can a significant amount of memory usage.

Instead, we can rely on Span to remove the allocations all together. Benchmarking a 3024 x 4032 image (the size my iPhone currently takes, so seems representative):

Before

Method	Mean	Error	StdDev	Gen0	Allocated
Skia_GetHash	23.08 ms	0.169 ms	0.158 ms	7750.0000	46.61 MB
ImageSharp_GetHash	22.93 ms	0.160 ms	0.150 ms	7750.0000	46.61 MB

After

Method	Mean	Error	StdDev	Allocated
Skia_GetHash	20.19 ms	0.162 ms	0.152 ms	1.21 KB
ImageSharp_GetHash	20.01 ms	0.047 ms	0.042 ms	1.29 KB

This effectively makes the memory usage 1.2 kilobytes regardless of the image size, down from 46 megabytes.

A smaller optimization is to place the hash in a stack buffer before converting it to hex. That just saves one small array allocation for the hash itself. This uses Convert.ToHexString since it can natively operate on a ReadOnlySpan<byte> and also uses upper-case lettering, but if you prefer I can create an overload of your extension method that works off of span as well.

Additionally, this fixes a tiny issue where IncrementalHash is not being disposed. This does not leak memory, but results in finalizers getting run during garbage collection.

[Webreaper]

This is totally awesome - thank you! I'll take a look later and merge.

That is a seriously good memory optimisation. 😁👌