A current rather untyped storage of options is one of the things that
contributes to the options code being so complex. This takes a small
step in synching down by storing the compiler options in dicts per
language.

Future work might be replacing the langauge strings with an enum, and
defaultdict with a custom struct, just like `PerMachine` and
`MachineChoice`.

This also fixes that the keys in ArgumentNode.kwargs are
all of the type BaseNode now. Before this commit, it was
possible that both strings and Nodes where used as keys.