dfa: fix approximate cache size

Unbelievably, this was using the size of the compiled prog *and* the
heap memory used by the cache to compute the total memory used by the
cache. The effect of this is that the reported size might be much bigger
than what is actually used by the cache. This in turn would result in
the lazy DFA thrashing the cache and going quite slow.

Author: BurntSushi

src/dfa.rs

@@ -1576,7 +1576,7 @@ impl<'a> Fsm<'a> {
     /// inputs, a new state could be created for every byte of input. (This is
     /// bad for memory use, so we bound it with a cache.)
     fn approximate_size(&self) -> usize {
-        self.cache.size + self.prog.approximate_size()
+        self.cache.size
     }
 }