Add std feature flag

[lucascool12]

Add std feature flag

[Dr-Emann]

At least the TreeMap type is backed by a BTreeMap, and things like serialize/serialize_into use Vec.

But it does seem like it wouldn't be too difficult to only require alloc, and provide a fully no-std build too.

[Dr-Emann]

Ah, one thing is our use of std::panic::catch_unwind, which isn't available outside of std, and there's not really a path currently to getting it in core/alloc AFAICS. I don't think there's a safe way to pass an arbitrary callback to C without it, but we could provide a std feature by default, and an alloc feature, and a completely no-std version without either.