Migrate from CIL's Pretty to OCaml's Format (initial pass)

[Copilot]

Begins the migration from CIL's unmaintained Pretty module to OCaml's standard Format module, which is faster (no intermediate doc construction, GADT-based internals), maintained, and compatible with ppx_deriving.show.

Core interface change

Added val pp: Format.formatter -> t -> unit to Printable.S — the de facto standard signature, also derived by ppx_deriving.show:

(* Before *)
val pretty: unit -> t -> Pretty.doc

(* After — both coexist during migration *)
val pretty: unit -> t -> Pretty.doc
val pp: Format.formatter -> t -> unit

Changes

• Printable.S — adds val pp: Format.formatter -> t -> unit
• printable.ml functors — all functors (SimplePretty, SimpleShow, SimpleFormat, UnitConf, HConsed, HashCached, LiftConf, Lift2Conf, ProdConf, Prod3, Liszt, Chain, LiftBot, LiftTop, Option) now derive/provide pp, so modules using them get it for free
• Domain modules — added pp to setDomain, mapDomain, hoareDomain, baseDomain, musteqDomain, threadIdDomain, intDomTuple, valueDomain, arrayDomain, structDomain, all apron domains, and utility modules (gobFpath, gobYojson, edge)
• VarType signature — added pp_trace: Format.formatter -> t -> unit with implementations in constrSys.ml, analyses.ml, node.ml
• Tracing system (goblint_tracing.ml, messages.ml) — rewrites gtrace/tracel/traceli to use Format.kfprintf/Format.ifprintf instead of Pretty.gprintf/GobPretty.igprintf; trace output is now written via a Buffer+formatter, then flushed as a string with Printf.eprintf
• 400+ trace call sites — bulk-updated to use D.pp instead of D.pretty, pp_trace instead of pretty_trace, and CilType.Exp.pp/CilType.Typ.pp/CilType.Lval.pp in place of CIL's d_exp/d_type/d_lval

Known remaining issues

A handful of call sites in td3.ml still use Pretty.docOpt (S.Var.pp_trace ()) and S.Dom.pretty_diff inside trace calls — these need Format-compatible replacements (GobFormat.pp_print_opt and a Format-based diff printer).

Messages.warn/info/debug/error still use Pretty-based format strings and are out of scope for this initial pass.

[sim642]

There's little use to this without doing this in CIL first. There are old branches in both that go 95% of the way (at the time). The reason they didn't get done is that there are tricky things that Pretty can do which are slightly different from Format.