[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 remember that this was discussed as I introduced `os_get_file_size`, but I couldn't find where.
I'm sure that my first version used `uint64_t` as libuv defines `st_size` as `uint64_t`, but I changed it back to `off_t`.
I would favor `uint64_t` instead of `size_t` though.