- eliminated traversing chain of all compunits loaded when deserializing a serialization context

- pre-allocate SCBody objects and store them in the weak hash (which is a little less weak now)
- make compunit loading from disk (by filename) threadsafe and atomic; add a loaded filename registry
- factor out some of the SC getting code from the two wval nqp:: ops in interp.c
work around lack of sc descriptions right now