vector: add comparison operators between pmem::vector and std::vector

add .gitattributes

Merge back changes from stable-1.5

extend vector API

set text and eol config parameteres

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.

add comparison operators between vector and std::vector

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

Change it in vector ctors and assign method.

backport: Disable some libcxx tests for old g++ compilers.

Reason: our default and move ctors are throwing

test: extend vector access tests

They cannot be compiled due to lack of full C++11 support.

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