Drop dependency on ahash 0.7.7

We used to have this (inverted) dependency structure:
ahash v0.7.7
└── hashbrown v0.12.3
    └── rkyv v0.7.44
        [dev-dependencies]
        └── hashbrown v0.14.3 (/home/ben/hashbrown)

rkyv's validation feature uses its alloc feature which uses hashbrown
0.12 with default features enabled, and one of those is ahash.

Cargo.toml

@@ -44,7 +44,7 @@ fnv = "1.0.7"
 serde_test = "1.0"
 doc-comment = "0.3.1"
 bumpalo = { version = "3.13.0", features = ["allocator-api2"] }
-rkyv = { version = "0.7.42", features = ["validation"] }
+rkyv = { version = "0.7.42", default-features = false, features = ["size_32"] }
 
 [features]
 default = ["ahash", "inline-more", "allocator-api2"]