Does not build in SBCL 2.0.9 - :type/:initform conflict

[quicklisp]

The latest SBCL will signal a full warning if the :initform of a slot is not of the type specified by the :type option.

This triggers a lot of warnings, and a build failure, in s-protobuf.

Full log here: http://report.quicklisp.org/2020-09-29/failure-report/s-protobuf.html#s-protobuf