scm_i_* in Snd

[Bill Schottstaedt]

> `scm_i_string_chars ()' is used by Mailutils, Lilypond, AutoGen, SND 

I'm not knowingly using any scm_i_* entities in Snd.  I removed the
scm_i_procedure_arity use, and I think the SCM_STRING_CHARS macro
is only used if it is defined.