BigInts for all the things

[atjn]

This PR is two-fold:

Use BigInts for ino IDs

As reported in #21, using Numbers for ino IDs is dangerous, because real ino IDs are 64-bit and can overflow the 32-bit Numbers, which can make the program believe that two different files are the same file, and then only count the size of one of them.

This change is theoretically just a minor fix, but if anyone uses a custom filesystem, it now has to support BigInts, which it did not have to do previously. Therefore, I think we should release this as v4.0.0 to avoid possible breakage.

Fixes #21

Support folder size output as BigInt

This change is arguably less important, and yet much more complex. It allows the user to choose that the size of the folder is returned as a BigInt. This is only necessary if you are working with crazy large folders, but there can be other practical reasons that you'd want the output as a BigInt.
It's a pretty common feature in other node libraries, and is also modeled to work the same way as other libraries, such as fs.

[atjn]

@alessioalex this is a major change, so I would like a thumbs up from you before I publish it :)

[alessioalex]

Looks great man, thank you!

Feel free to merge and publish it!