Add a `size()` function to WASI's `MetadataExt`.
Add a `size()` function to WASI's `MetadataExt`.

WASI's `filestat` type includes a size field, so expose it in
`MetadataExt` via a `size()` function, similar to the corresponding Unix
function.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust