Update dependencies to compile using Rust 1.64.

[tjni]

Some transitive dependencies were updated also, but the relevant ones are:

1. socket2 needs to be 0.3.16+ (due to Implement network primitives with ideal Rust layout, not C system layout rust-lang/rust#78802)
2. mdbook needs to be 0.4.21+ (due to "local_ctx does not live long enough" compilation error on rustc 1.64.0-nightly (62b272d25 2022-07-21) rust-lang/mdBook#1860)

The commands I ran:

cargo update -p socket2
cargo update -p mdbook --precise 0.4.21
cargo update -p futures-util