1) Changes the submodel flags to use a flagset instead of several booleans
2) Adds `rotate_accel` to allow submodel rotation acceleration to be specified
Submodel acceleration can now be instant; dumb-rotating submodels always instantly accelerate (and have always done so)
This adds the "no sparks" flag for subsystems which disables the
generation of spark sources for that subsystem when it is hit.
This was requested by Axem.
This introduces a new mechanism for handling bit flags. Instead of using an int (or two, or three) to write bits into, this uses the std::bitset in a wrapper class that accepts an enum class as input. This means that certain classes of errors we've had a few of in the engine, where a coder used the wrong set of flags or checked them against the wrong variable (SF_STEALTH vs SIF_STEALTH, for example) can now be detected at a compiler level rather than not at all.
Goober5000