Route: Only encode placeholders in type parameter

[tindzk]

Previously, static path elements were encoded in Route's HList. This prevented conditional routes such as the following:

if (isProduction) Root / "api" / "v2.0" else Root

Change the implementation such that only placeholders (e.g. Arg[String] or Fragment[Int]) are encoded in Route's type parameter.

Also, simplify the design by using tuples instead of HLists. As a consequence, the compile-time and run-time footprint is reduced.

Detailed changes:

• Use tuples instead of HLists
• Eliminate all type casts
• Drop all external dependencies (Shapeless, Cats)
• Argument and fragment placeholders are now encoded in Route's type parameter
• Change fragment delimiter from # to $
• Route parsing does not fail anymore when additional path elements are specified
• Drop ParamOpt[T] in favour of Param[Option[T]]
• Optional route arguments can now be populated with None instead of Option.empty[T]
• Remove all type aliases
• Custom Codecs can read/emit optional arguments

Closes #14. Closes #28.