Merge stable 1.5 into stable 1.6

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

doc: enable auto gh-pages generation in docker script

Create separate Docker images for stable-1.6 branch

tests: remove unnecessary #include <libpmemobj.h>

Merge changes from stable-1.5

allocator: remove unnecessary #include <libpmemobj.h>

Including libpmemobj.h causes inclusion of all type macros
which are not needed by C++.

Including libpmemobj.h causes inclusion of all type macros
which are not needed by C++.

Merge back changes from stable-1.5 to stable-1.6

[1.5] Do not check pkgconfig on windows

it might be installed (as part of cygwin).

Backport appveyor update to stable-1.5

... to reduce the Appveyor build time.

... to reduce the Appveyor build time.

PMEM_IS_PMEM_FORCE is not set in tests
and Appveyor build is timed out.

This change is required because of the following issue:
https://help.appveyor.com/discussions/problems/24684-problem-with-vcpkg-wrong-version-on-appveyor
https://github.com/appveyor/ci/issues/3052
which cannot be fixed directly now.

Extend warning message when valgrind is not found.

Install libpmemobj++.pc in all configurations

Specify we need devel package.

fix missing include header file

stdexcept is needed in slice.hpp for std::out_of_range support

[1.6] Mark allocation_flag constructor as explicit.

Having implicit constructor caused unexpected beahviour with
make_persistent_array. For make_persistent version designed to
work with sized arrays calling make_persistent<int[100]>(90) didn't
failed as expected. Instead, '90' was implicitly converted to
allocation_flag.

[1.6] Add option to force define IS_TRIVIALLY_COPYABLE

[1.6] string: fix max_size() return value

[1.6] Add missing include to v.hpp.

Needed by std::forward_as_tuple.