Introduce gdb::array_view

An array_view is an abstraction that provides a non-owning view over a
sequence of contiguous objects.

A way to put it is that array_view is to std::vector (and std::array
and built-in arrays with rank==1) like std::string_view is to
std::string.

The main intent of array_view is to use it as function input parameter
type, making it possible to pass in any sequence of contiguous
objects, irrespective of whether the objects live on the stack or heap
and what actual container owns them.  Implicit construction from the
element type is supported too, making it easy to call functions that
expect an array of elements when you only have one element (usually on
the stack).  For example:

 struct A { .... };
 void function (gdb::array_view<A> as);

 std::vector<A> std_vec = ...;
 std::array<A, N> std_array = ...;
 A array[] = {...};
 A elem;

 function (std_vec);
 function (std_array);
 function (array);
 function (elem);

Views can be either mutable or const.  A const view is simply created
by specifying a const T as array_view template parameter, in which
case operator[] of non-const array_view objects ends up returning
const references.  (Making the array_view itself const is analogous to
making a pointer itself be const.  I.e., disables re-seating the
view/pointer.)  Normally functions will pass around array_views by
value.

Uses of gdb::array_view (other than the ones in the unit tests) will
be added in a follow up patch.

gdb/ChangeLog
2017-09-04  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/array-view-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add array-view-selftests.o.
	* common/array-view.h: New file.
	* unittests/array-view-selftests.c: New file.