Minor problems with substring-related tag changes.

Minor problems with substring-related tag changes.
* symbols.h (SCM_SUBSTRP): Don't mask off the S bit; that's
exactly what we want to leave in to detect substrings.
(SCM_ROSTRINGP, ROUCHARS): Formatting tweaks.
* tags.h: Fix diagrams and comments describing the S tag bit;
remove vestigial remarks about the D tag bit.
(SCM_TYP7, SCM_TYP7S): Rephrased for readability.
* strings.c: Formatting tweaks.