rbyd: Dropped special altn/alta encoding

altas, and to a lesser extend altns, are just too problematic for our
rbyd-append algorithm.

Main issue is these break our "narrowing" invariant, where each alt only
ever decreases the bounds.

I wanted to use altas to simplify lfsr_rbyd_appendcompaction, but
decided it wasn't worth it. Handling them correctly would require adding
a number of special cases to lfsr_rbyd_appendrat, adding complexity to
an already incredibly complex function.

---

Fortunately, we don't really need altns/altas on-disk, but we _do_ need
a way to mark alts as unreachable internally in order to know when we
can collapse alts when recoloring (at this point bounds information is
lost).

I was originally going to use the alt's sign bit for this, but it turns
out we already have this information thanks to setting jump=0 to assert
that an alt is unreachable. So no explicit flag needed!

This ends up saving a surprising amount of code for what is only a
couple lines of changes:

           code          stack          ctx
  before: 38512           2624          640
  after:  38440 (-0.2%)   2624 (+0.0%)  640 (+0.0%)

scripts/dbgtag.py

@@ -183,13 +183,10 @@ def tagrepr(tag, w=None, size=None, off=None):
                 ' w%d' % w if w else '',
                 ' %s' % size if size is not None else '')
     elif tag & TAG_ALT:
-        return 'alt%s%s%s%s%s' % (
+        return 'alt%s%s 0x%03x%s%s' % (
                 'r' if tag & TAG_R else 'b',
-                'a' if tag & 0x0fff == 0 and tag & TAG_GT
-                    else 'n' if tag & 0x0fff == 0
-                    else 'gt' if tag & TAG_GT
-                    else 'le',
-                ' 0x%x' % (tag & 0x0fff) if tag & 0x0fff != 0 else '',
+                'gt' if tag & TAG_GT else 'le',
+                tag & 0x0fff,
                 ' w%d' % w if w is not None else '',
                 ' 0x%x' % (0xffffffff & (off-size))
                     if size and off is not None