BigInt offset typings

[dkourilov]

The field {offset} returned from OpenFile::fd_seek can be promoted to BigInt. Later on, in WASI::fd_seek the call to buffer.setInt32 fails for BigInt. The proposed patch fixes the problem.

[bjorn3]

Thanks for catching thie! Looks like the returned offset is in both cases unconditionally u64. Could you use setBigInt64 unconditionally in both cases and return only bigint and not number from the respective Fd methods?

[dkourilov]

Sorry I couldn't resist changing BigInt in types declarations to bigint interface primitive.

[bjorn3]

I don't really understand the difference between the two, but changing it is fine.

[bjorn3]

Thanks! Published as 0.2.10.

[dkourilov]

Thank you!