sv: major refactor

- Internal data pointer is read only by default => any uses that
  require use of the pointer itself (freeing, mutating) must perform a
  cast.

- refactor tests to use some new sv macros and functions, and clean
  them up.

- slight cleanup of sv.c

src/sv.c

@@ -58,8 +58,6 @@ sv_t sv_till(sv_t sv, const char *reject)
        ++offset)
     continue;
 
-  if (offset == sv.size)
-    return sv;
   return sv_truncate(sv, offset);
 }
 
@@ -73,8 +71,6 @@ sv_t sv_while(sv_t sv, const char *accept)
        ++offset)
     continue;
 
-  if (offset == sv.size)
-    return sv;
   return sv_truncate(sv, offset);
 }