Implement SIMPLE-FUN-P more efficiently
Put in a random comment about structure dumping
Fix grammar mistakes pointed out by Cyrus Harmon
Add two more known cases to checkgen

OBJECT-NOT-TYPE-ERROR 'FUNCTION -> OBJECT-NOT-FUN-ERROR, etc.
Implement COERCE-SYMBOL-TO-FUN more efficiently.

Rather than two globaldb inquiries we can do with just one.
(FDEFINITION is conceptually a globaldb inquiry)
As a side effect, this gives saner behavior in the perverse case
in which the compiler's understanding of function :KIND differs
from the runtime's reality.