Finding a procedure's arity uses binary search

Finding a procedure's arity uses binary search

* module/system/vm/assembler.scm (pack-arity-flags):
  (write-arity-headers): Add a flag to indicate that an arity is part of
  a case-lambda, so that we can use binary search to find arities.

* module/system/vm/debug.scm (is-in-case-lambda?)
  (arity-is-in-case-lambda?, find-first-arity): Use binary search.