hmac: add `Acceleration.Hmac`

* adapted from work on two previous client projects, I want to use it
  on a third so splitting it out as a proper library
* put tests in a new assembly so this could be worked on
  independently, and potentially split into a different git repo if
  that seems worthwhile
hmac: test hmac via msbuild
build: clean up nuget interaction

* rename `nugetPackage` to `package`
* new `push` target to run the nuget push
* determine nuget.exe binary like we do for others