modify slice to work with output iterators

basic_string: remove workaround to compile under VC++2015

After added conversion operators there is no need
to manually cast string to string_view. Casting in this case
produces VC++2015 errors due to operator==() ambiguity.

docker: add Ubuntu-18.04 docker image and test it in other_OSes.yml

Fix TBB cmake

string_view: VS2015 build fix

- remove class namespaces before class members which
  are in the same namespace to avoid compilation errors

- print more concise messages,
- make it required when TESTS_TBB is on.

string: unify description of find API

- change description to be the same for string and string_view

string_view: move find API implementation from string

- change function body to use conversion operator to string_view and
  call find*/rfind* function accordingly

- add find*, rfind* functions to string_view

Bump OSes versions

Don't use github.repository variable in workflow files

CI: use special environment var in GHA for Windows' PATH

Map example fix

So that hashmap is not always empty after reopen.

This reverts commit fc304b2e.

The old 'addPath' is depreciated now. For more details see:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

and enable additional package build in other_OSes.yml

we use them in other_OSes.yml - Ubuntu, because it's not LTS release,
Fedora, because PMDK rpm's don't work properly there.

string_view: implement ops API for string_view and port proper tests

result of make_persistent was assigned to a local variable instead
of to root()->pptr.

- add copy, substr, compare API

Do not run concurrent gdb tests under clang

On some versions of clang and gdb rbreak does not work correctly
for template function. More precisely, breakpoint at internal_insert_node
is never hit.

- taken from: llvm-mirror/libcxx@78d6a77

string_view: implement reverse iterators and tests

- change test implementation to test pmem::obj::string_view API

cmake: get rid of INSTALL_* options and always install all source

Update CONTAINER_REG variable to allow docker images push from forks