Add string optimizations

[Shane32]

This PR prevents PlainTextToBinaryByte from encoding the provided string back and forth to the target encoding multiple times. It also caches the encoder for future use, as it is designed to be a singleton. On the .NET Core 5+ TFMs the encoding uses a stack-allocated byte array, reducing heap allocations and memory requirements.

Add tests for UTF8 with and without BOM.

No changes to logic; produced QR codes should be identical to previously for any input.

[codebude]

Hello @gfoidl , hello @Shane32 - I would like to take this opportunity to say once again that I am very impressed by your suggestions and your discussion. To be honest, your optimizations reach a level that exceeds my knowledge of C#/.NET/JIT. Nevertheless, I think I vaguely understand what you are discussing and I think I have learned a thing or two. Thank you very much for your efforts. I bow my head!