- mapper compilation has been reorganized such that most compilation

- mapper compilation has been reorganized such that most compilation
  occurs upon mapper construction.  this allows us to have fewer
  calls to mapper.compile() and also to allow class-based properties
  to force a compilation (i.e. User.addresses == 7 will compile all
  mappers; this is [ticket:758]).  The only caveat here is that
  an inheriting mapper now looks for its inherited mapper upon construction;
  so mappers within inheritance relationships need to be constructed in
  inheritance order (which should be the normal case anyway).