Guards for wrapper types

[jackfirth]

Part of #42. Wrapper types should allow guards. The signature of a wrapper guard should match the contract (-> wrapper-type? any/c any/c), accepting an argument with information about the wrapper type (in order to use the type name in error messages, for instance) and an argument for the value being wrapped. It should return a single value representing the replacement value to wrap (possibly normalized to some canonical form, or wrapped with a chaperone, etc.).

Then, the make-wrapper-implementation function and the define-wrapper-type macro should both accept an optional #:guard parameter matching this contract. As a convenience, a (wrapper-guard/c contract) contract combinator should construct a function suitable for use with the #:guard option that simply checks the field with the contract.

Wrapper type guards can possibly be implemented more efficiently than tuple type guards (see #203), since they can assume exactly one field and there's no need to call apply anywhere in the code that translates tuple guards to struct guards.

[jackfirth]

Update: guard functions should probably match the contract (-> wrapper-type? (-> any/c any/c)), just to get any allocation related to error messages out of the way at type creation time.