BReference: add IsSet method

Change-Id: Ibc34dbde3b81faecdc7fe66a2c07fc33d7871ce2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3487
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

PVS V591: non void functions must return something

Change-Id: I1f4276bc32afab75cf857c143d2bdb4fc3b578b1
Reviewed-on: https://review.haiku-os.org/c/1612
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

BReference: missing const cast when building a BReference<const>

* The internal BReference is on a non-const object so it can increment
and decrement the reference count, so we need to cast the const away.

Whitespace cleanup only.

BReferencable: implement const references with specialization

This is simpler and cleaner than my previous attempt. Thanks to Ingo for
suggesting this.

Add a BConstReference class.

This is a BReference that allows only const access to the referenced
object. This was not easily possible with the existing BReference for
two reasons:
* BReference<const Type> would not work, as BReference needs to change
the reference count of the referenced object. Adding mutable and casting
where appropriate wouldwork but,
* It is now also possible to assign a BReference to a BConstReference
(to the same type, of course). The reverse is not allowed, making it
more difficult to "const cast" the referenced object (it's still
possible to "get" the object pointer and cast that).

BConstReference can be used to provide shared read-only access to an
object, for example this can be used to cache non-copiable or
expansive to create objects.

Make BReferenceable public.

* The Network Kit now makes use of it for BUrlContext, so we need this
in the public headers.
* Problem caught by the new build bot by compiling the unit tests.

BReference: missing const cast when building a BReference<const>

* The internal BReference is on a non-const object so it can increment
and decrement the reference count, so we need to cast the const away.

Whitespace cleanup only.

BReferencable: implement const references with specialization

This is simpler and cleaner than my previous attempt. Thanks to Ingo for
suggesting this.

Add a BConstReference class.

This is a BReference that allows only const access to the referenced
object. This was not easily possible with the existing BReference for
two reasons:
* BReference<const Type> would not work, as BReference needs to change
the reference count of the referenced object. Adding mutable and casting
where appropriate wouldwork but,
* It is now also possible to assign a BReference to a BConstReference
(to the same type, of course). The reverse is not allowed, making it
more difficult to "const cast" the referenced object (it's still
possible to "get" the object pointer and cast that).

BConstReference can be used to provide shared read-only access to an
object, for example this can be used to cache non-copiable or
expansive to create objects.

Make BReferenceable public.

* The Network Kit now makes use of it for BUrlContext, so we need this
in the public headers.
* Problem caught by the new build bot by compiling the unit tests.