Generics

[KacperFKorban]

Add basic support or generic types

e.g.

def greetMaybeName(maybeName: Option[String]): String =
  maybeName.fold("Hello!")(name => s"Hello, $name!")

@main
def run =
  guinep.web(greetMaybeName)

No support for generic functions yet. Also, generics with undecidable type params are not supported yet.

closes #24