Implements [Feature #19000]

This commit adds copy with changes functionality for `Data` objects
using a new method `Data#with`.

Since Data objects are immutable, the only way to change them is by
creating a copy. This PR adds a `with` method for `Data` class instances
that optionally takes keyword arguments.

If the `with` method is called with no arguments, the behaviour is the
same as the `Kernel#dup` method, i.e. a new shallow copy is created
with no field values changed.

However, if keyword arguments are supplied to the `with` method, then
the copy is created with the specified field values changed. For
example:
```ruby
    Point = Data.define(:x, :y)
    point = Point.new(x: 1, y: 2)
    point.with(x: 3) # => #<data Point x: 3, y: 2>
```

Passing positional arguments to `with` or passing keyword arguments to
it that do not correspond to any of the members of the Data class will
raise an `ArgumentError`.

Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>

Technically we shouldn't see Qfalse now, but RTEST also compiles down to
just one branch anyways. Pretty contrived issue, but easy to fix.

struct_ivar_get recently started using rb_class_superclass to
resolve super instead of RCLASS_SUPER. This change made
Qnil a possible case we need to return from within the
struct_ivar_get for loop.