[gdbsupport] Add gdb::array_view::{iterator,const_iterator}

[gdbsupport] Add gdb::array_view::{iterator,const_iterator}

While trying to substitute some std::vector type A in the code with a
gdb::array_view:
...
- using A = std::vector<T>
+ using A = gdb::array_view<T>
....
I ran into the problem that the code was using A::iterator while
gdb::array_view doesn't define such a type.

Fix this by:
- adding types gdb::array_view::iterator and gdb::array_view::const_iterator,
- using them in gdb::array_view::(c)begin and gdb::array_view::(c)end, as is
  usual, and
- using them explicitly in a unit test.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>