WIP: blast_bv tactic

This tactic eliminates any bitvector of (partially) known size in the local context, replacing it's usages with a sequence of `BitVec.concat`, with a new `Bool` added to the context for each bit.
This is a first step towards `bitblast`ing for bitvectors

src/Lean/Meta/Tactic/NormBv.lean

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+/-
+Copyright (c) 2024 Alex Keizer. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Alex Keizer
+-/
+-- prelude
+-- TODO: minimize imports
+import Lean.Meta.Tactic.Util
+import Lean.Elab.Tactic
+import Lean.Elab.Tactic.Basic
+import Lean.Meta.Tactic.Apply
+import Init.Data.BitVec
+
+namespace Lean.Meta
+
+open Lean.Elab.Tactic
+open Std BitVec
+
+@[elab_as_elim]
+theorem BitVec.concatCases
+    {motive : {w : Nat} → BitVec w → Sort u}
+    (nil    : motive BitVec.nil)
+    (concat : ∀ {w : Nat} (x : BitVec w) (b : Bool), motive (concat x b) ) :
+    ∀ {w} (x : BitVec w), motive x :=
+  sorry
+
+-- TODO: this is copied from `evalCases`, surely there's a better way to do this
+-- It's just doing the equivalent of `cases $target using BitVec.concatCases`
+def casesBitVec (target : Expr) : TacticM Unit := do
+  let elimInfo ← withMainContext <| getElimInfo ``BitVec.concatCases
+  let mvarId ← getMainGoal
+  -- save initial info before main goal is reassigned
+  let initInfo ← mkTacticInfo (← getMCtx) (← getUnsolvedGoals) (← getRef)
+  let tag ← mvarId.getTag
+  mvarId.withContext do
+    let targets ← addImplicitTargets elimInfo #[target]
+    let result ← ElimApp.mkElimApp elimInfo targets tag
+    let elimArgs := result.elimApp.getAppArgs
+    let targets ← elimInfo.targetsPos.mapM fun i => instantiateMVars elimArgs[i]!
+    let motiveType ← inferType elimArgs[elimInfo.motivePos]!
+    let mvarId ← generalizeTargetsEq mvarId motiveType targets
+    let (targetsNew, mvarId) ← mvarId.introN targets.size
+    mvarId.withContext do
+      ElimApp.setMotiveArg mvarId elimArgs[elimInfo.motivePos]!.mvarId! targetsNew
+      mvarId.assign result.elimApp
+      ElimApp.evalAlts elimInfo result.alts .missing #[] initInfo
+          (numEqs := targets.size) (toClear := targetsNew)
+
+syntax (name := blast_bv) "blast_bv" : tactic
+
+@[tactic blast_bv]
+partial def blastBv : Tactic := fun _ => loop where
+  loop : TacticM Unit := withMainContext <| do
+    let ctx ← getLCtx
+    let n ← mkFreshExprMVar (Expr.const ``Nat [])
+    let decl ← ctx.findDeclM? (fun decl => do
+      let isSucc ← isDefEq decl.type (.app (.const ``BitVec []) (.app (.const ``Nat.succ []) n))
+      let isZero ← isDefEq decl.type (.app (.const ``BitVec []) (.const ``Nat.zero []) )
+      if isZero || isSucc then
+        pure <| some decl
+      else
+        pure none
+    )
+    match decl with
+    | none => return ()
+    | some decl =>
+        casesBitVec (decl.toExpr)
+        loop
+        return
+
+syntax "blast_bv " "with" ident+ : tactic
+macro_rules
+  | `(tactic| blast_bv with $xs:ident*) =>
+      let xs := xs.reverse
+      `(tactic| blast_bv; rename_i $[$xs:ident]*)
+
+example (x : BitVec 2) (y : BitVec 2) : x + y = y + x := by
+  blast_bv with a a' b b';
+  /-
+  b'ba'a: Bool
+⊢ concat (concat nil a') b' + concat (concat nil a) b = concat (concat nil a) b + concat (concat nil a') b'
+  -/
+  sorry