Performance & General Housekeeping

[JustArion]

• Moves reused Guids to the static scope (Other files do this already)
• Removes some redundant type casts in NAudio.Wasapi
• Removes redundant backing field usages in NAudio.Wasapi

Feel free to cherry pick commits ;)
I would like to ask for some clarification on code style. Perhaps add a .editorconfig for future contributors. There's a lot of instances where var is used on simple types but then full type names are used in other places eg. int