Cleanup: Spelling, unused bindings

[bartelink]

(edit by @abelbraaksma):

This:

• fixes numerous spelling errors in comments and naming
• fixes "unused variable" warnings (this shows up in Rider, mostly)
• apply the new use _ = ... syntax, which is now supported by F#
• rename NUnit file to XUnit file (we don't use NUnit).

[abelbraaksma]

Cleaned up the commit history, this is now good to go.

Thanks for this, @bartelink, much appreciated!!!