fix: slicing a many-pointer yields a correct slice (issue 0159)

emitSubslice handled a struct (slice/string) base and an array base, but a
many-pointer [*]T base is an LLVM pointer kind — it fell through to the else arm
that mapped the result to LLVMGetUndef(slice_ty), so a slice of a many-pointer
(mp[lo..hi]) had a garbage .len/.ptr and iterating it segfaulted.

Add a LLVMPointerTypeKind branch: the base value IS the data pointer, so GEP by
lo and len = hi - lo (the caller supplies the bound; no length is read from the
unbounded pointer). An open-ended mp[lo..] has no resolvable upper bound (a [*]T
carries no length), so lowerSliceExpr now diagnoses it instead of emitting a
.length op that yields garbage.

A List (whose items is [*]T) is now iterable with for items[0..len] (e);
applied in Scheduler.deinit. Regressions: examples/types/0195 (valid slice +
List for-each) + examples/diagnostics/1192 (open-ended rejection).

library/modules/std/sched.sx

@@ -494,10 +494,8 @@ Scheduler :: struct {
         self.ready_tail = null;
 
         // (2) Free every heap Task allocated by `go`.
-        i := 0;
-        while i < self.task_allocs.len {
-            self.own_allocator.dealloc_bytes(self.task_allocs.items[i]);
-            i = i + 1;
+        for self.task_allocs.items[0..self.task_allocs.len] (t) {
+            self.own_allocator.dealloc_bytes(t);
         }
 
         // (3) Free the List backings (all grown through `own_allocator`).