typechecker: rename function "type" to fun_kind

Summary: when writing a "typechecker" program that "checks" "types",
 naming fields "type" creates the impression that their contents are
 the very "types" that are being checked, which is a "bad" "idea". More
 seriously, whether a function is async/sync/generator is pretty
 central to the typechecking that we do on its contents.

Reviewed By: msullivan, jwatzman

Differential Revision: D1463796