Snd 23.9:

s7: added optional let argument to immutable? and immutable!
    finally added error checks to the tree-* functions
      see lint.scm for the previous versions
    s7_make_c_pointer_wrapper_with_type
    ' (apostrophe) now is (#_quote ...) and similarly
      for the quasiquote helpers, apply-values, and list-values.

Thanks!: Norman Gray
