Add new Python APIs to support DAP value display

gdb's language code may know how to display values specially.  For
example, the Rust code understands that &str is a string-like type, or
Ada knows how to handle unconstrained arrays.  This knowledge is
exposed via val-print, and via varobj -- but currently not via DAP.

This patch adds some support code to let DAP also handle these cases,
though in a somewhat more generic way.

Type.is_array_like and Value.to_array are added to make Python aware
of the cases where gdb knows that a structure type is really
"array-like".

Type.is_string_like is added to make Python aware of cases where gdb's
language code knows that a type is string-like.

Unlike Value.string, these cases are handled by the type's language,
rather than the current language.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>