Introduce pmem::obj::experimental::string.

Add missing testcases for array.range()

Remove const_contiguous_iterator.

Reason: undefined behaviour

Implemented:
- constructors
- const methods for getting size/capacity
- begin/end
- operator[]/at

This iterator can be replaced by simple const pointer. To support
comparison operators between basic_contiguous_iterator,
range_snapshotting_iterator and pointers, conversion operator to const_pointer
was added. This change is a potential performance improvement (we don't
have another abstraction layer - const_contiguous_iterator). This also
allows to reduce amount of code.

vector assign API extension

vector: fix asserts which are always true

Fixes inspired by klockwork

test: do not open tx

Do not call methods in tx when not needed or when it is done internally

fix allocation_flag issues

aggregate initialization is allowed for structs with no
user-declared (explicitly defaulted or deleted constructors are
allowed) constructors since C++11, but gcc 4.8.5 doesn't support it

use 'gnutar' archive format for deb package

vector: non-member swap

test: fix pool's layout name

Our swap() implementation is throwing.

vector: add assign methods and operators

vector: rm active tx assert in vector API

verify if non-const access methods/iterators works out of tx scope

it is unsafe to use some access methods/operators (e.g. at(), begin())
out of transaction scope, however we can't forbid users to use them, because there is no other way
to modify elements without transaction

test: cover missing vector cases based on codecov analysis