Support running `utop` in repo root

[fsestini]

When working on a feature, it would sometimes be useful to fire up a utop REPL and run some experiments in there.

Currently this does not work, due to module name clashes:

$ dune utop
File "_none_", line 1:
Error:  Duplicated implementations:
         Multiple definitions of module Misc in files /Users/filses01/arm/herdtools7/_opam/lib/ocaml/compiler-libs/ocamlcommon.cma,
         lib/herdtools.cma,
         Multiple definitions of module Config in files /Users/filses01/arm/herdtools7/_opam/lib/ocaml/compiler-libs/ocamlcommon.cma,
         gen/common/gen_common.cma,
         Multiple definitions of module Parser in files /Users/filses01/arm/herdtools7/_opam/lib/ocaml/compiler-libs/ocamlcommon.cma,
         gen/common/gen_common.cma,
         Multiple definitions of module Location in files /Users/filses01/arm/herdtools7/_opam/lib/ocaml/compiler-libs/ocamlcommon.cma,
         lib/herdtools.cma

[fsestini]

CC @psafont

[psafont]

I had a PoC that created a new library at lib/base/, so Misc and Location can be moved there and exposed in a wrapped way. e.g. Base.Misc. Then a simple substitution, or a module Location = Base.Location before it's used would work well.

A similar thing for the modules in gen/common could be done, it's probably worth asking @ShaleXIONG to see if that wouldn't be too disruptive for him.