Support no_std builds

[dharjeezy]

Draft: blocked on ethereum_hashing#23, ethereum_serde_utils#21, ethereum_ssz#86 and tree_hash#53. CI here will fail dependency resolution until those are merged and released. Part of a series adding no_std support across the SSZ stack so it can be used from a Substrate runtime.

What this does

Adds no_std support. Nothing else.

This originally also added a ProgressiveList. That is dropped: #84 already covers EIP-7916 and is further along, so there is no sense in a competing type. This PR is now just the no_std work, as requested.

Why

Substrate runtimes compile to wasm32-unknown-unknown without std. The containers here need only core and alloc.

Changes

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

The std build is unchanged.

Why it cannot build yet

It depends on no_std versions of tree_hash, ethereum_ssz and ethereum_serde_utils, proposed in the PRs above and not yet released.

Verification

Verified locally against the proposed branches via path patches.

Check	Result
cargo test	67 + 3 passed
cargo check --no-default-features --features sha2 --target wasm32-unknown-unknown	clean
cargo fmt --check	no diffs

The branch name is left as it was so the existing review thread is preserved, and no longer describes the contents.

[michaelsproul]

there is another PR already under review for progressive list, if you'd like to do no_std please do just that in this PR

[dharjeezy]

Thanks, dropped it. This is now just the no_std change, and I have rebased so #84 stands on its own for EIP-7916.