src/c-types-{proto,impl,parse}.lisp: Add `storage specifiers' to the model.

src/c-types-{proto,impl,parse}.lisp: Add `storage specifiers' to the model.

A slightly unusual kind of thing.  This is intended to capture things
like storage classes and similar which are determinedly `top level',
unlike qualifiers which stay attached to the base type in the presence
of declarator operators.

Storage specifiers are attached to a special carrier type
`c-storage-specifiers-type' (with all of the necessary trimmings), and
there is new machinery (`wrap-c-type') to arrange that this stays at the
outermost level when deriving new types from existing ones.

There is a pile of additional protocol, mostly mirroring the main types
protocol, for dealing with storage-specifier syntax, in both Lisp and C.