feat(cli): add now builtin function

[omfj]

Adds a now() builtin function that returns the time since unix epoch in nanoseconds as SmallBigInt.

[omfj]

Also, is there a way for me to return a u128? I am using u64, because I couldn't figure out how to return a u128 as a BigInt.

[aapoalas]

Also, is there a way for me to return a u128? I am using u64, because I couldn't figure out how to return a u128 as a BigInt.

It might be that we're missing a from_u128(&mut Agent, NoGcScope) API for BigInt: do you want to try adding it? It should mostly be just a repetition of from_i64.

[aapoalas]

LGTM!