Making std optional.

[lvella]

This PR creates a new cargo feature std, on by default, that can be disabled if the user wants to target an environment with no support for std library.

Motivation

Crate is now supported in a larger number of targets, and is a step into fixing this issue: bluealloy/revm#493.

Solution

This crate doesn't really use any std feature (e.g., threads, IO, file, network). It is a pure computation library, and theoretically it could be pure #![no_std] with no need for a feature to enable "std". But unfortunately I hit into two issues:

• The fact that thiserror is currently std-only (no_std support on nightly dtolnay/thiserror#211), and the solution to make thiserror no_std depends on a unstable rust feature (Tracking Issue for Error in core rust-lang/rust#103765).
• Some floating point functions are only available in std (like log2, abs, pow, etc), this is an issue of LLVM relying on the system libraries implementation for these functions. This also happens with memcpy and integer division functions, which LLVM assumes to exist, and is solved by including these functions in compiler_builtins library that can be used by no_std projects. For some reason, the corresponding floating point functions are not in compiler_builtins yet (Tracking issue for f32 and f64 methods in libcore rust-lang/rust#50145). Thus, when std is disabled, this PR disables modules log, pow and root, along with From<f64> and From<f32> implementations, all which relies on the missing floating point functions.

PR Checklist

• [?] Added Tests
• [✓] Added Documentation
• [✓] Updated the changelog

[prestwich]

The fact that thiserror is currently std-only (dtolnay/thiserror#211), and the solution to make thiserror no_std depends on a unstable rust feature (rust-lang/rust#103765).

Yes, you can't use thiserror, you have to implement core::fmt::Display manually, and then std::error::Error behind a cfg flag. It's a bit of a pain but not so bad.

[Wizdave97]

The fact that thiserror is currently std-only (dtolnay/thiserror#211), and the solution to make thiserror no_std depends on a unstable rust feature (rust-lang/rust#103765).

Yes, you can't use thiserror, you have to implement core::fmt::Display manually, and then std::error::Error behind a cfg flag. It's a bit of a pain but not so bad.

@lvella @prestwich mind if I finish this up since we need it urgently on our end

[gakonst]

Superseded in #274, we'll get it done today. Thank you.