ml-kem/module-lattice: `integer_extend_truncate` warnings

[tarcieri]

Just started getting these warnings:

warning: an associated function with this name may be added to the standard library in the future
   --> ml-kem/src/algebra.rs:123:27
    |
123 |         let N = start_n + u8::truncate(i);
    |                           ^^^^^^^^^^^^
    |
    = help: call with fully qualified syntax `truncate(...)` to keep using the current method
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = note: `#[warn(unstable_name_collisions)]` (part of `#[warn(future_incompatible)]`) on by default
help: add `#![feature(integer_extend_truncate)]` to the crate attributes to enable `core::num::<impl u8>::truncate`
   --> ml-kem/src/lib.rs:45:1
    |
 45 + #![feature(integer_extend_truncate)]

It seems we should at least switch to using Truncate::truncate as the invocation syntax, but also this seems like a case where we should perhaps consider a different API entirely, or at least a different method name, so as not to clash with the standard library.