chore(Agglayer): refactors to make scaling down easier

[mmagician]

Two small refactors described in #2331 (comment):

• the test vectors should generate a uint string with amounts, not a hex string
  • this PR is using U256 internally-only, while still keeping only two constructors: new(bytes: [u8; 32]) & from_uint_str(s: &str)
• the amount & destination address should be extracted to stack before validate_claim, saved to memory locals, and passed on stack "as arguments" to the build_p2id_output_note procedure

[partylikeits1983]

Looks great! I'm glad we added back the primitive-types U256 dependency :)