1.0.30.38: faster TRUNCATE on floats

1.0.30.38: faster TRUNCATE on floats

 * Specialized %UNARY-TRUNCATE/SINGLE-FLOAT and
   %UNARY-TRUNCATE/DOUBLE-FLOAT.

 * Explicit coercions to appropriate float types in the TRUNCATE
   transforms. This gets rid of generic arithmetic in the general case
   (Python is reluctant to insert explicit integer-tofloat coercions
   for integers of unknown range due to precision issues.)

 * Since COERCE (and %SINGLE-FLOAT and %DOUBLE-FLOAT) are not
   flushable, take core not to generate leftover code in the TRUNCATE
   transform when the result lvar has a single-value type.

 * Rename %UNARY-TRUNCATE float VOPs, so that transforming to a
   specialized floating point version doesn't make use unable to
   implement it directly as a VOP when the range of the float is
   sufficiently constrained.