src/method-{proto,impl}.lisp: Add `:valist' method-entry role.

src/method-{proto,impl}.lisp: Add `:valist' method-entry role.

For each varargs message, add a new entry whose name has a `__v' suffix,
and which takes a `va_list' argument in place of the variable-length
argument list of the `nil' entry.  The `nil' entry now just sets up the
`va_list' pointer and invokes the corresponding `:valist' entry function.

This actually makes constructing the method-entry functions somewhat
cleaner, since the handling of variable argument lists is now
concentrated in the construction of a separate entry function.