[Rust] Fix nightly build

[jaudiger]

Building on nightly is not possible due to not having the upstream ahash bug fix from tkaitchuck/aHash#183:

error[E0635]: unknown feature `stdsimd`
  --> /Users/jaudiger/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ahash-0.8.6/src/lib.rs:99:42
   |
99 | #![cfg_attr(feature = "stdsimd", feature(stdsimd))]
   |

On the current main branch, we have two ahash dependencies:

• one on version 0.8.6
• the other on version 0.7.7

The resolution of the bug was done in the version 0.8.7: https://github.com/tkaitchuck/aHash/releases/tag/0.8.7.

In order to build brioche on nightly, I did two things (separated in two commits):

• First commit is to get rid of the old ahash 0.7.7 dep by updating deno_ast to 0.28.0: https://github.com/denoland/deno_ast/releases/tag/0.28.0
• Second commit is to only update the dep ahash 0.8.6 without updating anything else with cargo update ahash

With both changes, brioche can be built on the latest Rust nightly version 2024-07-08.

[kylewlacy]

LGTM