Ignore .scala_dependencies

This appears to be some sort of file generated by the Scala compiler.
I'm not entirely sure what it does or why it appears, but it
occasionally does.

Regardless, I'm pretty sure it shouldn't be checked in.
Add a dummy map phase

This doesn't really do anything, but it contains much of the
infastructure that would be user during a proper mapping phase.  Right
now it will "work" for any circuits where Chisel just happens to
generate a legal mapping (which is defined as all narrow operations)
but fail whenever anything more complicated shows up.