Replace core dependency with base + stdio

[hhugo]

Summary

Removes the dependency on Jane Street's core, replacing it with the much lighter base + stdio (+ sexplib0). This shrinks the dependency footprint and build time, which is particularly relevant for the js_of_ocaml (stancjs) build.

• Dependencies: core → base, stdio, sexplib0. The ppx_jane bundle is replaced with only the rewriters actually used: ppx_base, ppx_pipebang, ppx_sexp_message, ppx_sexp_value, ppx_expect, ppx_inline_test.
• Mechanical changes: open Base instead of open Core; I/O goes through Stdio; stdlib modules that Base deprecates (Format, Obj, Scanf, Marshal, …) are qualified with Stdlib.
• Core-only APIs replaced:
  • String.{Map,Set,Table,Hash_set} → Base's comparator-passing style (Map.M(String).t, Set.of_list (module String), Hashtbl.create (module String), …)
  • Expr.Typed's Map/Set submodules (previously from Core.Comparable.S, which Base does not provide) are now small explicit submodules exposing exactly the operations used.
  • Map.Make_using_comparator, Set.of_map_keys, fst3 → Base equivalents.
• New module Common.Poly_containers: Base's Set.Poly/Map.Poly lack sexp converters, so this provides sexp_of_t for both in Core's exact output format — which is why no expect-test output changed.
• The Generated_signatures code generator now emits Base.Hashtbl instead of Core.String.Table.

Notes for reviewers

• ppx_jane silently included ppx_pipebang (rewrites x |> f to f x at parse time), and the codebase depends on it: e.g. |> UnsizedType.UArray (constructor after pipe) and several places where record/constructor disambiguation only typechecks with the pipe inlined. It is kept as an explicit dependency.
• Two tiny semantic substitutions: elt ** 2. → elt *. elt (Base makes ** integer exponentiation) and Operator.of_string_opt builds its Sexp.Atom directly instead of going through the deprecated Sexp.of_string.

Testing

• dune build @all clean
• dune runtest (unit expect tests + integration cram suite): all pass without a single expected-output change
• dune build @runjstest (stancjs under node): passes
• Smoke-tested the stanc binary end-to-end on an integration model

🤖 Generated with Claude Code

[WardBrian]

Thanks @hhugo. I ended up going with a similar solution in #1643, but one that touches fewer files in the process. Helping trim down the list of what was actually needed (especially on the ppx side) was a huge assist