[RFC] FileInfo getters for `filesize`, `hardlinks` and `blocksize`

This implements some simple getters for `FileInfo`. (continues #619)

There are still some attributes wich get accessed through the `FileInfo.stat` struct, but I would like to implement some better abstractions for them in other PR's (like #775). The goal is to make the `uv_stat_t stat` struct private.
> I would favor uint64_t instead of size_t though.

yep