Don't delete the previous symbol table entry when shadowing subs.

Don't delete the previous symbol table entry when shadowing subs.

Instead just overwrite the CODE slot within. That prevents the GV from being
freed, which totally throws off the tokenizer, that holds a pointer to the gv
already.

This makes t/method.t and others work under the debugger, although I have no
idea why it ever worked, even with the debugger disabled.