Support no_std builds

[dharjeezy]

Draft: blocked on ethereum_hashing#23 and ethereum_ssz#86. 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 previously also added merkle proof and multiproof generation. That is split out per your request and will come as its own PR once this lands, so each can be judged on its own. The diff here drops from 1583 additions to 32.

Why

Substrate runtimes compile to wasm32-unknown-unknown without std. Everything this crate needs is in 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

7 files, 32 insertions, 20 deletions. The std build is unchanged.

Why it cannot build yet

tree_hash depends on ethereum_hashing with the sha2 feature, which #23 adds, and on ethereum_ssz no_std from #86.

Verification

Verified locally against the proposed branches via path patches.

Check	Result
cargo test	54 + 28 + 6 passed
cargo check --no-default-features --features sha2 --target wasm32-unknown-unknown	clean
cargo fmt --check	no diffs

One caveat: the trybuild test cannot pass under a path patch, since it spawns its own cargo build that does not inherit the patch and resolves ethereum_hashing from crates.io. It should pass normally once #23 releases.

Where this sits

1. ethereum_hashing#23 and ethereum_serde_utils#21
2. ethereum_ssz#86
3. this PR
4. ssz_types#85

Proof generation follows separately after this. The branch name is unchanged so the review thread is preserved, and no longer describes the contents.

[michaelsproul]

one feature per PR please

[dharjeezy]

Understood, split. This PR is now the no_std change only.

The proof generation and multiproof work is held back and will come as its own PR once this lands, so it can be judged separately