[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.
> st_size is uint64_t so that's another reason to favor size_t.

Not really, it's a reason to favour `uint64_t`. `size_t` is 32-bits on 32-bits systems, yet files can and do grow larger than 4GB there.