Is this library safe for crypto?

[AsafFisher]

Functions such as to_signed_bytes_be are safe?

[dignifiedquire]

could you clarify what you mean by "safe"?

[AsafFisher]

Sometimes functions can have side effects that can cause side channel attacks... I am not an expert but I've heard of these... did anyone do a review of this for this library?

Look at BigInt impl in golang, they walk about it there.