Fix pernicious bug of symbols without vts.
Fix pernicious bug of symbols without vts.

This bug was introduced in 8e1aaf898f39d8a0b4a512273fe5b8b886a6b1e6.

In it, I changed the primitive table to map from Symbol to function.

However, before loading core (and thus, before loading Symbol), we end
up creating Symbols for the primitives, and they end up with invalid vts.

thiago-silva/memetalk,memetalk/memetalk,thiago-silva/memetalk,memetalk/memetalk,memetalk/memetalk,thiago-silva/memetalk