Improve code generated for segment variable matching in SYNTAX-RULES.

Improve code generated for segment variable matching in SYNTAX-RULES.
Before it used an idiom of

  ((LET ((LOOP #F)) (SET! LOOP (LAMBDA () ...)) LOOP)
   ...),

which caused spurious & confusing `Possible inapplicable operator #f'
warnings from the compiler's flow analyzer.  Now it uses

  ((LET () (DEFINE LOOP (LAMBDA () ...)) LOOP)
   ...),

like the code generated for named LET.