Unify signed bit extraction via gadget

[hero78119]

We have signed bit extraction implementation in various place, especially in some gadget
E.g.

• SRAI
• InnerSignedLtConfig

  ceno/ceno_zkvm/src/gadgets/is_lt.rs

  Lines 337 to 350 in e3ce193

  	let is_lhs_neg = IsLtConfig::construct_circuit(
  	cb,
  	|| "lhs_msb",
  	max_signed_limb_expr.clone(),
  	lhs.limbs.iter().last().unwrap().expr(), // msb limb
  	1,
  	)?;
  	let is_rhs_neg = IsLtConfig::construct_circuit(
  	cb,
  	|| "rhs_msb",
  	max_signed_limb_expr,
  	rhs.limbs.iter().last().unwrap().expr(), // msb limb
  	1,
  	)?;

• msb_decompose

  ceno/ceno_zkvm/src/uint/arithmetic.rs

  Line 298 in e3ce193

  pub fn msb_decompose<F: SmallField>(

By switch to SignedExtendConfig introduced in this PR #455 we can save wit from 2 -> 1 with same lookup cost (1 bit + 1 limb check)