sv: add a few more functions

* alisp.org: reset state of Unit tests to TODO

* alisp.org: take sv_t tasks out of the backlog and set to WIP

* sv: add prototypes for sv_chop_{left,right} and sv_substr

* sv: implement sv_chop_{left, right} and sv_substr

* sv: sv_till and sv_while

* sv: add sv_truncate

When you just want to decrease to a specific size, sv_chop_right is a
bit cumbersome.

* alisp.org: Update and adjust

src/sv.c

@@ -22,6 +22,62 @@ sv_t sv_copy(sv_t old)
   return SV(newstr, old.size);
 }
 
+sv_t sv_chop_left(sv_t sv, u64 size)
+{
+  if (sv.size <= size)
+    return SV(NULL, 0);
+  return SV(sv.data + size, sv.size - size);
+}
+
+sv_t sv_chop_right(sv_t sv, u64 size)
+{
+  if (sv.size <= size)
+    return SV(NULL, 0);
+  return SV(sv.data, sv.size - size);
+}
+
+sv_t sv_substr(sv_t sv, u64 position, u64 size)
+{
+  return sv_chop_right(sv_chop_left(sv, position), size);
+}
+
+sv_t sv_truncate(sv_t sv, u64 newsize)
+{
+  if (newsize >= sv.size)
+    return sv;
+  return SV(sv.data, newsize);
+}
+
+sv_t sv_till(sv_t sv, const char *reject)
+{
+  if (sv.size == 0 || !sv.data)
+    return SV(NULL, 0);
+
+  u64 offset;
+  for (offset = 0; offset < sv.size && strchr(reject, sv.data[offset]) == NULL;
+       ++offset)
+    continue;
+
+  if (offset == sv.size)
+    return sv;
+  return sv_chop_right(sv, sv.size - offset);
+}
+
+sv_t sv_while(sv_t sv, const char *accept)
+{
+  if (sv.size == 0 || !sv.data)
+    return SV(NULL, 0);
+
+  u64 offset;
+  for (offset = 0; offset < sv.size && strchr(accept, sv.data[offset]) != NULL;
+       ++offset)
+    continue;
+
+  if (offset == sv.size)
+    return sv;
+  return sv_chop_right(sv, sv.size - offset);
+}
+
 /* Copyright (C) 2025, 2026 Aryadev Chavali
 
  * This program is distributed in the hope that it will be useful, but WITHOUT