Stringify process

i'm indoubt, but i added it to the PR since @jouvin requested it
i think a use case that shouldn't be generalised to an extra sub in `CAF::Process` is a more convincing argument.

i thought there was at some point a request to be able to check if the executable (i.e. the first elemnt of the `COMMAND` list) existed, and this would require access to this list, but one could also argue that this is valid enough to create a `executable_exists` sub in `CAF::Process`.