Provide the C WASI implementation as an option.

This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.

wasmtime-wasi-c/sandboxed-system-primitives/src/random.h

@@ -0,0 +1,20 @@
+// Part of the Wasmtime Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://github.com/CraneStation/wasmtime/blob/master/LICENSE for license information.
+//
+// Significant parts of this file are derived from cloudabi-utils. See
+// https://github.com/CraneStation/wasmtime/blob/master/lib/wasi/sandboxed-system-primitives/src/LICENSE
+// for license information.
+//
+// The upstream file contains the following copyright notice:
+//
+// Copyright (c) 2016 Nuxi, https://nuxi.nl/
+
+#ifndef RANDOM_H
+#define RANDOM_H
+
+#include <stdint.h>
+
+void random_buf(void *, size_t);
+uintmax_t random_uniform(uintmax_t);
+
+#endif