rename file to make earlier tab-complete.
Various changes to fix dump_registers.

The important one was establishing the fxsave area, as noted in the
comment.

Also changed callback signature to take third argument (of type `*()`)
in an attempt to ensure that it will always be safe to tail-call from
a call to `dump_registers` over to the callback.
Added debug flags during debugging of sro-play.rs.

Not sure they actually buy me anything here (they did not seem to help
in tracking down the `dump_registers` overwrite issue).
Updated sro-play to match new signature of Registers::dump.