`mapping.get_sequence(...).as_str_list()` is a very common
pattern seen both in plugins and the core. Adding a helper to reduce
the number of operations will make usage smoother

Remove completely '_yaml.dump()' and replace all notions and call by
'roundtrip_dump'

One difference is that 'MappingNode.items()' does not strip the
provenance from scalars and lists, which ends up not affecting the
code much.

Part of #1043

Avoid confusion by not referring to starting another process as
'spawning'. Note that 'spawn' is a process creation method, which is an
alternative to forking.

Say 'create child process' instead of 'fork' where it doesn't harm
understanding. Although we currently only use the 'fork' method for
creating subprocesses, there are reasons for us to support 'spawn' in
the future.

More information on forking and spawning:
https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Calls to `isinstance` can be particularily costly. Using type() is
much faster. The only known case where the `isinstance` was useful
is for dictionnaries where we would ask for a 'Mapping' instead.

Disallowing 'Mapping' for expected_type considerably speeds up the
calls to this functions.

Also add into NEWS

Now that we have both artifact and source caches the documentation
is updated to reflect that. Some sections headings/links etc. have been
expanded and changed.

Part of #1025