src/c-types-impl.lisp: Make string -> simple-c-type mapping more useful.

src/c-types-impl.lisp: Make string -> simple-c-type mapping more useful.

  * Arrange for `define-simple-c-type' to accept multiple C type-name
    strings, and keep records in the `*simple-type-map*' allowing simple
    type names to be canonified.

  * Add a new function `find-simple-c-type' to retrieve the appropriate
    type object given one its names.

  * Use this when looking up types in property values.

  * Remove the type declarations from the builtin module because they're
    all entered as simple types directly now.