Tidy-up of attributes and pool classes:
* Bring design up to date.
* New function PoolHasAttr encapsulates attribute checking.
* Abstract classes are abstract and mustn't be checked.
* The dummy pool class in fotest needs a size.
* Abstract pool classes null out methods that they can't provide a generic implementation for, to force subclasses to provide one.
* New function PoolTrivFramePopPending provides a generic implementation of that method.
* Rename PoolNoFreeWalk to PoolTrivFreeWalk since it has NOOP rather than NOTREACHED.
* Check that AttrMOVINGGC implies AttrGC.
* Remove unimplemented attributes (BUF_RESERVE, BUF_ALLOC, INCR_RB, INCR_WB, PM)
* AMC now inherits from AMCZ instead of the other way round. This is simpler: AMC adds features to AMCZ rather than AMCZ taking features away (and not quite getting it right).
* Similarly, LO inherits from AbstractSegBufPoolClass + PoolClassMixInCollect so that it doesn't have to clear AttrSCAN and the scan methods.
* Fix bug in MFSCheck -- mustn't check unroundedUnitSize >= UNIT_MIN since small unit sizes are rounded up to UNIT_MIN.
* Don't see AttrFREE in MRG (since no free method is supplied).
* Check AttrSCAN systematically (in PoolScan and SegCheck) rather than opportunistically in TraceStart and TraceQuantum.