Redesign Kotlin verb "interface"

[alecthomas]

Currently verbs have to be in a class and there can be multiple. This is a bit weird given how we want verbs to be seen as stateless single units.

Proposal is to make verbs top-level functions, rather than member functions of a module class. Modules will be declared with a file-level annotation, like in Go. The module name will be derived from the package name (as is the case presently).

@file:Module
package ftl.exemplar

import xyz.block.ftl.Module
import xyz.block.ftl.Verb

data class Request(val value: String)
data class Response(val status: String)

@Verb
fun something(context: Context, req: Request): Response {
  ...
}