Optimise FlatDictionary init for scenarios where no values are stored and llow inlining access

[Joannis]

This provides specialized access to the type, and virtually eliminates its presence from routes where a flat dictionary is not used. This affects the route parameters primarily, but helps in particular because CoreRequestContext always starts out with an empty Parameters instance.

[adam-fowler]

Empty arrays don't allocate any memory, so I'm not sure this is going to be a saving, as you have added a branch whenever accessing the parameters now

[Joannis]

I think my measurements are macOS-specific then. Swift arrays do seem to have a different (default) implementation from the ContiguousArray one

[adam-fowler]

@Joannis can I close this?