Setting to always use `/` separate paths on windows

Functions like `invocation_directory()` will return a `\`-separated path on windows, and functions like `join` will join path components with `/` on windows. This causes problems when using bash as the shell, since `\` will be interpreted as escape sequences.

We can't change this in a backwards compatible way, so we could introduce a setting, something like `set slash-paths` or something, which always uses `/` in paths and paths functions.

This setting would also make `invocation_directory()` not depend on cygpath on Windows.