SI-8126 Add a '-Xsource' flag allowing compilation in e.g. 2.10 mode

This is aimed at letting the 2.11+ compiler operate, at least until
phase:typer, on a 2.10 codebase.
SI-8126 Puts SI-6899 fix under a source level flag
SI-8126 Puts SI-7335 fix behind a source level flag
Merge pull request #3340 from huitseeker/issues/sourceFlag

SI-8126 adds a -source option to let (parts of) the compiler run on older code