[khmer] Rewrite most of shaper to better follow spec

Khmer spec has only one reordering phase, and only simple prebase
matra and Coeng-Ro reordering.  Implement that.  Specifically,
this was done to address recognizing different orders of the matra
and Coeng-Ro sequence.  That said, some combinations are now
reordered differently from Uniscribe.  Not clear if that's intended
or a bug in Uniscribe.  The following two sequences render the same
in Uniscribe whereas we reorder them differently:

  U+17A0,U+17D2,U+179A,U+17C2
  U+17A0,U+17C2,U+17D2,U+179A

For that reason, our test suite numbers regressed slightly.  Used
to be at 34 for fails, now at:

KHMER: 299080 out of 299124 tests passed. 44 failed (0.0147096%)

But generally a good change, and removed lots of code.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1026

src/hb-ot-shape-complex-indic-private.hh

diff --git a/src/hb-ot-shape-complex-indic-private.hh b/src/hb-ot-shape-complex-indic-private.hh
index 9554994..bb7fff3 100644
--- a/src/hb-ot-shape-complex-indic-private.hh
+++ b/src/hb-ot-shape-complex-indic-private.hh
@@ -300,7 +300,9 @@
   0x0CB0u, /* Kannada */
   0x0D30u, /* Malayalam */	/* No Reph, Logical Repha */
 
-  0x0DBBu, /* Sinhala */		/* Reph formed only with ZWJ */
+  0x0DBBu, /* Sinhala */	/* Reph formed only with ZWJ */
+
+  0x179Au, /* Khmer */
 };
 
 static inline bool