Functions: begin(), end(), free_data(), size(), capacity()
are being used in ctors/dtor and tests.
This commit provides these functions as well.
Index access operators are added.

Imported from llvm-mirror/libcxx@5ae92f0

Modify pmem::obj::array tests

vector: fix _alloc() error handling

We need to cache pmemobj_tx_alloc return value and only after that
assign it to _data, because when pmemobj_tx_alloc fails, it aborts
transaction.

test: fix vector default ctor test

Even if we are removing created files in our test framework,
we should delete manually persistent pointers (for clarity sake).

it was not build previously

check for brace initialization in new expression for specific classes

This line will fail to compile with clang++ <=7.0.0 for C++17
right after destructor will be added to vector class.

Check for issues related to agregate initialization in new expression
for specific classes. Set flag in order to avoid unsafe code in tests.

vector: fix functions description

vector: rm unnecessary cast

Merge back changes from stable-1.5

Change conversion operator from T() to T&() in v<>

persistent_ptr: add unsigned index operator 1.5

Ref: #103

Vector skeleton

Merge back changes from stable-1.5

Fix v swap, assignment operators and constructors.

Fix include in one more header

Fix headers inclusion for system headers 

Merge back changes from stable-1.5

Fix headers inclusion for system headers

Add info about pmemcheck annotations in readme

Fix ambiguous comments in make_persistent array

Changes are applied to files that were created after stable-1.5