Support no_std builds

[dharjeezy]

Part of a five crate series adding no_std support across the SSZ stack, so it can be used from a Substrate runtime. This one has no dependencies on the others and is ready to review. The order is ethereum_hashing#23 and ethereum_serde_utils#21 first, then ethereum_ssz#86, then tree_hash#53, then ssz_types#85. The later three are drafts because they cannot resolve until these two are released.

What this does

Adds no_std support, so the crate can be used from a wasm runtime.

Why

Substrate runtimes compile to wasm32-unknown-unknown without std. The serde helpers here need only core and alloc, so the change is mostly swapping std imports for alloc ones and gating the crate.

Changes

• #![cfg_attr(not(feature = "std"), no_std)] with extern crate alloc
• std imports replaced with alloc equivalents where they are used unconditionally
• the std feature wires through to the dependencies that need it

The std build is unchanged.

Verification

Check	Result
cargo test	40 passed
cargo check --no-default-features --target wasm32-unknown-unknown	clean
cargo fmt --check	no diffs

A note on the diff size

Two of the three commits are unrelated housekeeping that the tooling flagged while working here: one applies rustfmt, one fixes current clippy lints. They are what makes this touch 16 files rather than a handful.

They are kept as separate commits, so if you would rather review the no_std work alone, say so and I will drop them and open them separately, or not at all.