explicitly cast values to match parameters

travis: run valgrind tests only in one configuration.

and run tests with valgrind only in one configuration on travis.

Explicit cast is required for persistent_ptr (array type overload)
operator[] which now takes integral_type, but after merge back
from stable 1.5 will require std::ptrdiff type.

test: port miscellaneous libcxx's vector tests

Reason: our default and move ctors are throwing

test: extend vector access tests

vector: add modifiers methods.

string: use const_pointer as  const_iterator

Add insert(), emplace(), push_back(), erase() methods and their
overloads.

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