Add a way to disable default layout definitions.

[timothee-haudebourg]

When a type is defined in TreeLDR, it also defines a default layout for this type. This is very useful sometimes, but not all the time. We should add a way to disable this behavior. I see two ways of doing this:

• Generate by default, disable explicitly:

// Foo is a type and a layout.
type Foo {
  ...
}

// Bar is just a type, not a layout.
type Bar {
  ...
} nolayout;

• Disable by default, generate explicitly:

// Foo is just a type, not a layout.
type Foo {
  ...
}

// Bar is a type and a layout.
type layout Bar {
  ...
}