Slim base64

A simple and short no_std no_alloc Rust base64 implementation that is easy to audit (180LOC), with reasonable performance.

Why?

I need base64 encoding/decoding in a highly security critical project of mine, and I realised that auditing thousends of lines of the well known base64 crate, and reauditing it every time it updates would take me more time than implemneting it on my own. The resulting single file implementation is 180LOC, so you can also audit it in a resonable amount of time and vendor it in easly.