Add new Python APIs to support DAP value display

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>