Tidy up and publish picotoml

[IsaacWoods]

picotoml is a tiny no_std TOML parser that we've found useful for config parsing in Seed etc. Looking at crates.io, this niche doesn't actually seem to be filled by an existing crate very well, so we should publish picotoml in case other projects find it useful.

• Switch peeking iterator to use smallvec to avoid heap allocs if not needed
• Clean up general quality of the code
• Write better readme and publish
• Run a fuzzer over it at some point