1.0.43.67: COERCE: don't trust vector dimensions in unsafe code

1.0.43.67: COERCE: don't trust vector dimensions in unsafe code

 Fixes bug 655872.

 Our deftransform for COERCE takes advantage of ANSI's allowance
 to generate faster code, and open codes

  (COERCE X '(SIMPLE-VECTOR 5))

 in a way that doesn't verify the length of the simple-vector.

 1. Previously we did that for SAFETY < 3, but that doesn't really
    fit with our general policy, so enable it only for SAFETY = 0.

 2. Make the corresponding DERIVE-TYPE optimizer aware of this, so
    that it can drop the dimensions from the type when necessary.