Reduce allocations around the use of ArgSlice

[PaulusParssinen]

This PR includes various little refactoring and changes that can be considered "peanut-butter" as Toub terms them.

^{I have shared my concerns about this type in general on Garnet discord already so I'm not gonna repeat them here 😄}

Commits

• Make ArgSlice.Bytes a method and rename it to ToArray
  • This makes it clearer to the API consumers that this action is not free. This To-prefix pattern may seem familiar from the BCL.
• Mark all properties on ArgSlice as readonly
• Remove unnecessary allocations for ArgSlice length checks
  • It indeed was not clear whether accessing Bytes is cheap. Use Length property directly instead and avoid the unnecessary allocation.
• Remove misc. allocations relating to use of ArgSlice
• Switch some ArgSlice initializations to use constructor instead of initializer expression
• Use the public ArgSlice.Length property instead of the internal field where applicable