[confidential-transfer] Simplify ciphertext arithmetic for confidential transfers

[samkim-crypto]

Problem

The add_with_lo_hi() and subtract_with_lo_hi() functions chain primitive operations that each perform complete deserialize-compute-serialize cycles. Intermediate results are serialized then immediately deserialized by the next operation, creating redundant overhead. Each call performs 5 deserializations and 3 serializations when only 3 and 1 are required respectively—representing significant serialization and deserialization overhead.

These functions are invoked on every confidential balance update: subtract_with_lo_hi debits the sender, add_with_lo_hi credits the receiver's pending balance, and add_with_lo_hi is called again when applying pending balances. A single end-to-end confidential transfer invokes these operations 3-4 times, compounding the overhead across every confidential token movement in the system.

Summary of Changes

Simplified the ciphertext artihmetic to avoid redundant serialization.

[joncinque]

Looks great!