IrreversibleMigration: Ignore methods that are part of another method call

[AlasdairWallaceMackie]

Reason for change

Resolves issue #48

Consider the following migration:

Sequel.migration do
  change do
    alter_table(:stores) do
      add_column(:products, JSON, null: false, default: Sequel.pg_json({}))
      add_constraint(
        :only_one_user,
        (
          Sequel.cast(Sequel.~(user_id: nil), Integer) +
          Sequel.cast(Sequel.~(owner_id: nil), Integer)
        ) => 1,
      )
    end
  end
end

In the current state, the following offenses would be registered:

Sequel/IrreversibleMigration: Avoid using "pg_json" inside a "change" block. Use "up" & "down" blocks instead.
Sequel/IrreversibleMigration: Avoid using "+" inside a "change" block. Use "up" & "down" blocks instead.
Sequel/IrreversibleMigration: Avoid using "cast" inside a "change" block. Use "up" & "down" blocks instead.
Sequel/IrreversibleMigration: Avoid using "~" inside a "change" block. Use "up" & "down" blocks instead.
Sequel/IrreversibleMigration: Avoid using "cast" inside a "change" block. Use "up" & "down" blocks instead.
Sequel/IrreversibleMigration: Avoid using "~" inside a "change" block. Use "up" & "down" blocks instead.

Methods that are part of another method argument can be safely ignored, because an offense will still be registered if the parent method is invalid within a change block