Add first class types

[solnic]

Introduce Drops.Type with:

• Drops.Type - primitive types list :string, :integer etc.
• Drops.Type.Sum - composition of two types
• Drops.Type.List - a :list type with a member_type
• Drops.Type.Schema - previous Schema module converted into a standard built-in type

Now a schema looks like this:

contract.schema() #=> %Drops.Type.Schema{
  primitive: :map,
  constraints: [predicate: {:type?, :map}],
  keys: [
    %Drops.Type.Schema.Key{
      path: [:age],
      presence: :required,
      type: %Drops.Type{
        primitive: :integer,
        constraints: [predicate: {:type?, :integer}]
      }
    },
    %Drops.Type.Schema.Key{
      path: [:name],
      presence: :required,
      type: %Drops.Type{
        primitive: :string,
        constraints: [predicate: {:type?, :string}]
      }
    }
  ],
  atomize: false
}