Implement WASI C API.

This commit implements an initial WASI C API that can be used to instantiate
and configure a WASI instance from C.

This also implements a `WasiBuilder` for the C# API enabling .NET hosts to bind
to Wasmtime's WASI implementation.

crates/c-api/include/wasi.h

@@ -0,0 +1,70 @@
+// WASI C API
+
+#ifndef WASI_H
+#define WASI_H
+
+#include "wasm.h"
+
+#ifndef WASI_API_EXTERN
+#ifdef _WIN32
+#define WASI_API_EXTERN __declspec(dllimport)
+#else
+#define WASI_API_EXTERN
+#endif
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define own
+
+#define WASI_DECLARE_OWN(name) \
+  typedef struct wasi_##name##_t wasi_##name##_t; \
+  WASI_API_EXTERN void wasi_##name##_delete(own wasi_##name##_t*);
+
+// WASI config
+
+WASI_DECLARE_OWN(config)
+
+WASI_API_EXTERN own wasi_config_t* wasi_config_new();
+
+WASI_API_EXTERN void wasi_config_set_argv(wasi_config_t* config, int argc, const char* argv[]);
+WASI_API_EXTERN void wasi_config_inherit_argv(wasi_config_t* config);
+
+WASI_API_EXTERN void wasi_config_set_env(wasi_config_t* config, int envc, const char* names[], const char* values[]);
+WASI_API_EXTERN void wasi_config_inherit_env(wasi_config_t* config);
+
+WASI_API_EXTERN bool wasi_config_set_stdin(wasi_config_t* config, const char* path);
+WASI_API_EXTERN void wasi_config_inherit_stdin(wasi_config_t* config);
+
+WASI_API_EXTERN bool wasi_config_set_stdout(wasi_config_t* config, const char* path);
+WASI_API_EXTERN void wasi_config_inherit_stdout(wasi_config_t* config);
+
+WASI_API_EXTERN bool wasi_config_set_stderr(wasi_config_t* config, const char* path);
+WASI_API_EXTERN void wasi_config_inherit_stderr(wasi_config_t* config);
+
+WASI_API_EXTERN bool wasi_config_preopen_dir(wasi_config_t* config, const char* path, const char* guest_path);
+
+// WASI instance
+
+WASI_DECLARE_OWN(instance)
+
+WASI_API_EXTERN own wasi_instance_t* wasi_instance_new(
+  wasm_store_t* store,
+  own wasi_config_t* config,
+  own wasm_trap_t** trap
+);
+
+WASI_API_EXTERN const wasm_extern_t* wasi_instance_bind_import(
+  const wasi_instance_t* instance,
+  const wasm_importtype_t* import
+);
+
+#undef own
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // #ifdef WASI_H