Increase size of ref_counts and add assertions

This bumps the size of the reference counts from 16- to 32-bit counters to make
it less likely to overflow. Also assert in the retain function that the
reference count didn't overflow.

32-bits seems big enough for non-pathological examples but a more fool-proof
fix may be to bump it to 64-bits.

src/runtime/tree.h

@@ -45,7 +45,7 @@ typedef struct Tree {
     TSLexMode lex_mode;
   } first_leaf;
 
-  unsigned short ref_count;
+  uint32_t ref_count;
   bool visible : 1;
   bool named : 1;
   bool extra : 1;