Fix invalid read caused by commit 963b7b9. Whoops.
symtab: Fix seed scan to allow for tuples.
symtab: Class init should use a smart rule to determine default sigs.

Every once in a while, I add a new type, and the conversation with myself goes
something like this:

"Okay, adding this new type should be easy."

...

"Huh, I set it to ...integer, integer and now it's string, integer"

...

"Weird, it's like all of the sigs are the sa...OH NOT AGAIN."