rename m+ to m+/lenient and make m+ more strict

Adding two quantities with different dimensions is still a run-time
error unless they require the result to have the expected dimension.
For example:
> (m+ (make-measure 1 meter) (make-measure 1 second))
- : (measure (U Complex (Vectorof Real)) (U Length-Unit Time-Unit)
Sig-Figs)
. . m+: can't add two measures with different dimensions
  given (m 1 meter) and (m 1 second)