Improve schema generation performance on large schemas

Avoid SUBTYPEP during macroexpansion, it can be very expensive on SBCL.
Actually recognize LIST-OF:LIST-OF constructs.
In DEFINE-SCHEMA, don't nest the DEFMETHODs inside the LET,
this also can cause SBCL's control flow analysis to blow up.

Work done with Steven Spitz for QPX.

Tested: (asdf:test-system :cl-protobufs)