Add bigint type

[barnjamin]

We should consider adding a new type bigint that is backed by bytes and type check for that on b+,b*,b-, ...

Maybe here? maybe not if we want to keep that isolated to just the actual AVM types
https://github.com/tinymanorg/tealish/blob/main/tealish/tealish_builtins.py#L8

[barnjamin]

sketched out an idea here #63