Add (back) constructor for Errno

[Thomasdezeeuw]

Title says it all, would you consider adding back a constructor similar to Error::from_raw_error used to be?

Required for getrandom crate: https://github.com/rust-random/getrandom/blob/157d6f23dc53ea995294646c4f6301116aaa2bfa/src/error.rs#L113-L115.

[newpavlov]

Relevant comment: #59 (comment)

[alexcrichton]

This crate has now upgraded to WASI 0.2.0 which is much different from the previous wasi_snapshot_preview1 module. In that sense this issue is no longer applicable as this is for a historical version of the crate at this point. Because of that I'm going to close this.