1.0.6.8: improve (SUBSEQ FOO 0 ...) on arrays with elements >= word-size

1.0.6.8: improve (SUBSEQ FOO 0 ...) on arrays with elements >= word-size

	* check for a constant-valued start parameter and pass its value
	  through to the loop-generating machinery.  Before we would have
	  compared 'START to 0, which always fails; now we can recognize
	  that case and generate better code.