[set] Change pagesize from 512 bits to 8192 bits

Fixes perf regression on some heavy fonts in Chrome's FT+HB
interaction.

See:
https://bugs.chromium.org/p/chromium/issues/detail?id=782220

More work to be done:
https://bugs.chromium.org/p/chromium/issues/detail?id=794896

src/hb-set-private.hh

@@ -151,7 +151,7 @@ struct hb_set_t
       return 0;
     }
 
-    static const unsigned int PAGE_BITS = 512; /* Use to tune. */
+    static const unsigned int PAGE_BITS = 8192; /* Use to tune. */
     static_assert ((PAGE_BITS & ((PAGE_BITS) - 1)) == 0, "");
 
     typedef uint64_t elt_t;