wasi_snapshot_preview1 recently requires fd_readdir to return actual
inode values. On zero, wasi-libc will call fdstat to retrieve them.
This introduces our own `platform.Dirent` type and `Readdir` function
which a later change will allow fetching of inodes.
See https://github.com/WebAssembly/wasi-libc/pull/345
Signed-off-by: Adrian Cole <adrian@tetrate.io>