Replace `-Z symbol-mangling-version=v0` with `-C symbol-mangling-version=v0`.

Replace `-Z symbol-mangling-version=legacy` with
`-Z unstable-options -C symbol-mangling-version=legacy`.

This commit modifies how instances are polymorphized so that closures
and generators have any closures or generators captured within their
upvars also polymorphized - this avoids symbol clashes with the new
symbol mangling scheme.
Signed-off-by: NDavid Wood <david@davidtw.co>