Add optional_param with enums

Nobody has claimed this yet.

This sets up some enums so you can have type safe params

```crystal
class Tasks::Index
# `allow` will create an enum
param filter, default: All, allow: [All, Completed, Incomplete]

action do
# This will be extra cool/if when they have exhaustiveness checks on enums!
case Filter
when All
tasks = TaskQuery.all
when Completed
tasks = TaskQuery.new.completed
when Incomplete
tasks = TaskQuery.new.incomplete
end

render tasks: tasks
end
end

# You can only use things in the enum when using a route
link "complete", to: Tasks::Index.route(filter: Tasks::Index::Filter::Completed)
```

Start with the issue's Crystal examples for `optional_param`, the `allow` enum declaration, and route generation. Determine how enum values should be represented in params and routes, then define completion around type-safe filtering and restricting route links to the declared enum values.

Written by the indexing model from the issue text.