Simplfy the mutable array story:

	- rename MutableArray to STArray (and similarly
	  for all operations on MutableArray, eg newArray
	  is now newSTArray).

	- remove the extra level of indirection between
	  STArrays and MutableArrays.

	- remove the MutableArray interface from
	  hslibs/lang/MutableArray.  This module will go
	  away soon - Andy, don't bother porting it to Hugs.