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/js-polyfill/polyfill.c

@@ -0,0 +1,45 @@
+#include <emscripten.h>
+#include "wasmtime_ssp.h"
+#include "../src/posix.h"
+
+static __thread struct fd_table curfds_pointee;
+
+int main(int argc, char *argv[]) {
+    return 0;
+}
+
+void handleFiles(void) {
+    struct fd_table *curfds = &curfds_pointee;
+
+    fd_table_init(curfds);
+
+    // Prepopulate curfds with stdin, stdout, and stderr file descriptors.
+    if (!fd_table_insert_existing(curfds, 0, 0))
+        __builtin_trap();
+    if (!fd_table_insert_existing(curfds, 1, 1))
+        __builtin_trap();
+    if (!fd_table_insert_existing(curfds, 2, 2))
+        __builtin_trap();
+
+    EM_ASM(" \
+        const imports = { wasi_unstable: WASIPolyfill }; \
+        let file = document.getElementById('input').files[0]; \
+        let file_with_mime_type = file.slice(0, file.size, 'application/wasm'); \
+        let response = new Response(file_with_mime_type); \
+        wasi_instantiateStreaming(response, imports) \
+        .then(obj => { \
+            setInstance(obj.instance); \
+            try { \
+                obj.instance.exports._start(); \
+            } catch (e) { \
+                if (e instanceof WASIExit) { \
+                    handleWASIExit(e); \
+                } else { \
+                } \
+            } \
+        }) \
+        .catch(error => { \
+            console.log('error! ' + error); \
+        }); \
+    ");
+}