Add make-load-form for sb-kernel:lexenv

[Paul F. Dietz]

I have a use case where I'd like to save the lexical environment constructed during compilation. For file compilation, this means the lexenv must be storable into the fasl file, so it needs a make-load-form.

This is complicated by the presence of FUNCTION objects in the one or more of the slots, for example MACROS (where they come from macrolet). However, we don't need the exact same function at load time, just something that computes the same thing. So, the load form could be constructed by taking the source of these functions and having an equivalent lambda or named lambda be found in the load form.