Move .pmc to xattr

[rurban]

Advantage:

• faster stat (only one, plus getxattr). see https://engineering.shopify.com/235340559-bootsnap-optimizing-ruby-app-boot-time
• kept together with the source file.
• will be moved with the source file.

Relies on platform-compat File::ExtAttr, esp. Windows is missing, though trivial to add. Needs a fallback when user_xattr is not mounted. (See http://linux.goeszen.com/extended-file-attributes-on-your-filesystem-how-to-enable-and-use-them.html) use File::Attributes::Extended.