DEV: Replace params by the contract object in services

This patch replaces the parameters provided to a service through
`params` by the contract object.

That way, it allows better consistency when accessing input params. For
example, if you have a service without a contract, to access a
parameter, you need to use `params[:my_parameter]`. But with a contract,
you do this through `contract.my_parameter`. Now, with this patch,
you’ll be able to access it through `params.my_parameter` or
`params[:my_parameter]`.

Some methods have been added to the contract object to better mimic a
Hash. That way, when accessing/using `params`, you don’t have to think
too much about it:
- `params.my_key` is also accessible through `params[:my_key]`.
- `params.my_key = value` can also be done through `params[:my_key] =
  value`.
- `#slice` and `#merge` are available.
- `#to_hash` has been implemented, so the contract object will be
  automatically cast as a hash by Ruby depending on the context. For
  example, with an AR model, you can do this: `user.update(**params)`.

app/controllers/admin/users_controller.rb

@@ -121,10 +121,10 @@ class Admin::UsersController < Admin::StaffController
 
   def suspend
     User::Suspend.call(service_params) do
-      on_success do |contract:, user:, full_reason:|
+      on_success do |params:, user:, full_reason:|
         render_json_dump(
           suspension: {
-            suspend_reason: contract.reason,
+            suspend_reason: params.reason,
             full_suspend_reason: full_reason,
             suspended_till: user.suspended_till,
             suspended_at: user.suspended_at,