Support wasm32-wasi compile target

[dunnock]

Compiling to wasm32-wasi on the stable rustc 1.39 fails, so it seems solution implemented in #84 is not working anymore.

Currently compilation fails:

   Compiling net2 v0.2.33 (https://github.com/rust-lang-nursery/net2-rs#eda403f0)
error[E0432]: unresolved import `libc::__wasi_fd_t`
  --> /Users/max/.cargo/git/checkouts/net2-rs-4b5319289a3d25c1/eda403f/src/sys/wasi/mod.rs:12:25
   |
12 | use libc::{self, c_int, __wasi_fd_t};
   |                         ^^^^^^^^^^^ no `__wasi_fd_t` in the root

error[E0432]: unresolved import `sys::c::__wasi_fd_t`
 --> /Users/max/.cargo/git/checkouts/net2-rs-4b5319289a3d25c1/eda403f/src/sys/wasi/impls.rs:5:17
  |
5 | use sys::{self, c::__wasi_fd_t};
  |                 ^^^^^^^^^^^^^^ no `__wasi_fd_t` in `sys::c`

There are now implementations of a networking in wasi which hopefully could be used to provide networking.

There are various dependent crates which would be great to compile to WASI, like async-rs/async-std#505