Define condition variable-used-in-multiple-accumulations

[robert-strandh]

• In the file Code/Loop/compile-time-conditions.lisp, define a new
  condition VARIABLE-USED-IN-MULTIPLE-ACCUMULATIONS. It should have
  slots for the variable, and two accumulation types.

• In the file Code/Loop/condition-reporters-english.lisp, define a
  condition reporter, modeled after the others in the same file,
  specialized to the new condition type. The message should be the one
  that is currently in the call to ERROR in the file
  Code/Loop/analysis.lisp, in the function
  CHECK-ACCUMULATION-CATEGORIES.

• In the file Code/Loop/analysis.lisp, in the function
  CHECK-ACCUMULATION-CATEGORIES, replace the existing call to ERROR by a
  call to ERROR using the new condition.