Implement Ops.get_* shortcut methods

In code geenreated by Y2R, we often have pieces like this:

  Convert.to_string(Ops.get(l, 0, "foo"))

This code is typically emitted because the original collection is
untyped.

Because this pattern appears over and over, we decided to simplify it to
something like this:

  Ops.get_string(l, 0, "foo"))

That is, introduce and use shortcut methods that would combine getting
an item from a collection with type conversion.

This commit adds needed shortcut methods to Ruby bindings. It will be
followed by a commit in Y2R that will generate code which will use them.