Add document describing requirements for containers

update RELEASE_STEPS.md

- cleanup what's not needed
- define what $VERSION and $VER mean
- clean up some sentences and add announcements on slack channel

Update CI files, incl. coverage-related fixes

now it explicitly says it executes debug job, with
additional COVERAGE variable set.

Change in upstream repo may (and did) break our coverage. It's best to use fixed version,
with a patch applied (incl. removal of an excessive log, due to the log limit in CI).

- exit 1 if not successful
- add prints and update comments
- check for gcov leftovers; codecov's script should parse and remove them

Extend error handling in allocator<void>::allocate

Merge changes from stable 1.10 to master

align issues' labels to recent update

Update PMDK version to 1.9 in CI

tests: enable map.ops tests which require generic std::less

add CONTRIBUTING.md file

Intead of std::less<void> which is available from C++14 use
transaprent less which has the same implementation.

Leave just one test with std::less<> as sanity check.

Enablement of generic erase test

- Add helper header for map-like structures
- Enable erase_key test

to include latest pmreorder fixes.

Merge stable 1.9 into stable 1.10

get rid of old WA for vcpkg and use latest release instead

force removal of docs' dir since it might not exist yet

LIBCXX map tests: OPS

Add ops tests taken from libcxx master branch

get rid of old WA for vcpkg and use latest release instead

add stable-1.10 to valid-branches.sh

add document describing release steps

Add headers taken from libcxx