Use int64 for fields that require it

[iljya]

Golang's int is 32 or 64 bits depending on the platform so int64 should be used explicitly to ensure these fields don't overflow.

I did not put much thought into backwards compatibility.

Golang int notes: https://go.dev/tour/basics/11 Telegram API: https://core.telegram.org/bots/api

[negasus]

Same as #11 , merged