Currently, the use of the downcast method means that you have to use one of the hard-coded types. But Enarx needs to define its own `WasiFile` implementations. This works fine, except the resulting files cannot be used in poll because they aren't part of the hard-coded list. Replace this with an accessor method for the pollable type in `WasiFile`. Because we provide a default implementation of the method and manually implement it on all the hard-coded types, this is backwards compatible. Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>
WASI implementation in Rust, using cap-std.