[longterm] explore pushing support further down to 4.01 by relying on Ast_mapper

[hyphenrf]

note that the ppxlib api (and especially metaquot) should preferably remain the main development vector, just that for older compilers we can use dune-configurator (?) to target an included file where the metaquots are expanded and ppxlib functions are replaced.

it is apparent that this would be an awful lot of work, but the components seem decoupled enough that it won't be too much of an architecture effort, just lots of plumbing.