`#![no_std]` support?

[davnavr]

Currently, I don't see any attributes that declare #![no_std] support in lib.rs, which may prevent #![no_std] code from depending on varint-simd.

With #![no_std] , x86-64 intrinsics would still be available at core::arch::x86_64, and from my quick search of the source code, varint-simd doesn't depend on alloc or any other std APIs that core provides.

The small amounts of code that do rely on the standard library (e.g. impl std::error::Error for VarIntDecodeError) could be conditionally compiled with an std feature, similar to other #![no_std] crates.