Ensure that SHA2 and SHA3 crates are #![no_std]

[jjkurczak]

Summary:

The purpose of this sub-issue is to implement sufficient no_std changes such that SHA2 and SHA3 crates are able to be built, tested, and benchmarked in no_std.

Scope:

The expected changes for this initial task may involve changes to other crates, and the workspace, to support required changes to crate dependencies, and to create a baseline for all crates to implement the new functionality uniformly.

Acceptance Criteria:

• A new feature will be added that positively enables std, which will be on by default
• the --no-default-features flag will be used to enforce no_std at build time
• SHA2 and SHA3 crates must build/test/bench, individually, with --no-default-features
• All crates must continue to build/test/bench, with no regressions, in default config.
• Building other crates or the entire workspace with --no-default-features is unsupported, with no required behavior associated with acceptance of this sub-issue