Add support for wasi_snapshot_preview1. (#592)

* Add support for wasi_snapshot_preview1.

This adds support for the new ABI, while preserving compatibility
support for the old ABI.

* Fix compilation on platforms where nlink_t isn't 64-bit.

* rustfmt

* Fix Windows build errors.

crates/test-programs/tests/wasm_tests/runtime.rs

@@ -33,7 +33,7 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
 
     // Create our wasi context with pretty standard arguments/inheritance/etc.
     // Additionally register andy preopened directories if we have them.
-    let mut builder = wasi_common::WasiCtxBuilder::new()
+    let mut builder = wasi_common::old::snapshot_0::WasiCtxBuilder::new()
         .arg(bin_name)
         .arg(".")
         .inherit_stdio();
@@ -48,11 +48,13 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
     let (reader, _writer) = os_pipe::pipe()?;
     builder = builder.stdin(reader_to_file(reader));
 
+    // The current stable Rust toolchain uses the old `wasi_unstable` ABI,
+    // aka `snapshot_0`.
     module_registry.insert(
         "wasi_unstable".to_owned(),
         Instance::from_handle(
             &store,
-            wasmtime_wasi::instantiate_wasi_with_context(
+            wasmtime_wasi::old::snapshot_0::instantiate_wasi_with_context(
                 global_exports.clone(),
                 builder.build().context("failed to build wasi context")?,
             )