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

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

stable1.5: fix persistent ptr

This reverts commit 6bbb319e.

This reverts commit 0f0779d7.

Decrease number of persistent_ptr dereferences by caching raw pointer.

Ref: #181

Fix argument parsing in examples/map_cli.

Fix range_snapshotting iterator when snapshot_size == 0

When snapshot_size == 0, no snapshotting should happen. This condition
must be explicitly checked inside range_snapshotting_iterator.

Parameters to iterator should specify range accessible by this
iterator. Before this patch those arguments described the whole
array instead of just the part from start to start + n.

argn was not incremented in case of get(). This resulted
in "invalid queue operation" message on every get.

Also fix "usage" for "insert" operation.

Ref: #186

Fix typo

common: fix typo

tests: change initialization of slice

gcc (7.3.0) gives array-subscript error when pointer to array is
decremented by 1.

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

persistent_ptr: add unsigned index operator 1.5

Ref: #103

Fix v swap, assignment operators and constructors.

Fix include in one more header

Fix headers inclusion for system headers

Assignment operators didn't work correctly because operatorT()
was not really a const function and this_type (used in those functions)
was not decalared. This resulted in compile error while trying to
assign v<> of different types.

Additionally v<> constructor taking const T& as parameter is not
needed. Using it would mean assigning some value to underlaing val
in v<>, but first call to get() would run constructor (which would
potentially change the value).

Fix typo in pmem::obj::array method description

Add informations about type limitations to persistent_ptr

and specify what is considered as UB by the standard.

Fix include in contiguous_iterator.hpp