Update rust toolchain to v1.81.0

[mohanson]

Updated to be consistent with ckb https://github.com/nervosnetwork/ckb/blob/develop/rust-toolchain.toml

Note that cargo gave a warning after the update, so I added a lints config in Cargo.toml as cargo suggested.

--> src/machine/mod.rs:1:7
  |
1 | #[cfg(has_asm)]
  |       ^^^^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
  = help: consider using a Cargo feature instead
  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
           [lints.rust]
           unexpected_cfgs = { level = "warn", check-cfg = ['cfg(has_asm)'] }
  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(has_asm)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default

Avoid to use legacy numeric method

```text
error: usage of a legacy numeric method
  --> src/lib.rs:62:14
   |
62 |         u64::max_value(),
   |              ^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#legacy_numeric_constants