implementation cleanup: merge slt & sltu opcode circuit

[hero78119]

The config can be merged similar as in mulh.rs

ceno/ceno_zkvm/src/instructions/riscv/mulh.rs

Lines 135 to 149 in 1f5b990

	enum MulhSignDependencies<E: ExtensionField> {
	UU {
	constrain_rd: IsEqualConfig,
	},
	SU {
	rs1_signed: Signed<E>,
	rd_signed: Signed<E>,
	constrain_rd: IsEqualConfig,
	},
	SS {
	rs1_signed: Signed<E>,
	rs2_signed: Signed<E>,
	rd_signed: Signed<E>,
	},
	}