Proof of concept C host

TODO deduplicate with add.wasm see #333

```sh
cd component-model/examples/example-c-host
gcc -o host host.c -lwasmtime && ./host 6 7 adder.wasm
```

Co-authored-by: Shelley <shelley@exe.dev>

Author: ohAitch

component-model/examples/example-c-host/.gitignore

@@ -0,0 +1 @@
+host

component-model/examples/example-c-host/adder.wasm

@@ -0,0 +1 @@
+../example-host/add.wasm

component-model/examples/example-c-host/host.c

@@ -0,0 +1,127 @@
+/**
+ * C host for the adder WebAssembly component.
+ *
+ * Uses the Wasmtime C API's component model support to load and run
+ * a component that exports: docs:adder/add.add(u32, u32) -> u32
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <wasmtime.h>
+
+static void exit_if_error(const char *step, wasmtime_error_t *error) {
+  if (error == NULL)
+    return;
+  wasm_byte_vec_t error_message;
+  wasmtime_error_message(error, &error_message);
+  wasmtime_error_delete(error);
+  fprintf(stderr, "error: failed to %s\n%.*s\n", step, (int)error_message.size,
+          error_message.data);
+  wasm_byte_vec_delete(&error_message);
+  exit(1);
+}
+
+int main(int argc, char *argv[]) {
+  if (argc != 4) {
+    fprintf(stderr, "Usage: %s <x> <y> <component.wasm>\n", argv[0]);
+    return 1;
+  }
+
+  uint32_t x = (uint32_t)atoi(argv[1]);
+  uint32_t y = (uint32_t)atoi(argv[2]);
+  const char *path = argv[3];
+
+  // 1. Create engine with component model enabled
+  wasm_config_t *config = wasm_config_new();
+  wasmtime_config_wasm_component_model_set(config, true);
+  wasm_engine_t *engine = wasm_engine_new_with_config(config);
+
+  // 2. Read the component file
+  FILE *f = fopen(path, "rb");
+  if (!f) {
+    fprintf(stderr, "error: could not open %s\n", path);
+    return 1;
+  }
+  fseek(f, 0, SEEK_END);
+  long fsize = ftell(f);
+  fseek(f, 0, SEEK_SET);
+  uint8_t *wasm_bytes = malloc(fsize);
+  fread(wasm_bytes, 1, fsize, f);
+  fclose(f);
+
+  // 3. Compile the component
+  wasmtime_component_t *component = NULL;
+  exit_if_error("compile component",
+      wasmtime_component_new(engine, wasm_bytes, fsize, &component));
+  free(wasm_bytes);
+
+  // 4. Create linker and add WASI P2
+  wasmtime_component_linker_t *linker =
+      wasmtime_component_linker_new(engine);
+  exit_if_error("add WASI to linker",
+      wasmtime_component_linker_add_wasip2(linker));
+
+  // 5. Create store with WASI config
+  wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
+  wasmtime_context_t *context = wasmtime_store_context(store);
+  exit_if_error("set WASI config",
+      wasmtime_context_set_wasi(context, wasi_config_new()));
+
+  // 6. Instantiate
+  wasmtime_component_instance_t instance;
+  exit_if_error("instantiate component",
+      wasmtime_component_linker_instantiate(linker, context, component, &instance));
+
+  // 7. Look up the exported "add" function.
+  //    The export is nested: first find the "docs:adder/add@0.1.0" instance,
+  //    then the "add" function within it.
+  wasmtime_component_export_index_t *iface_idx =
+      wasmtime_component_instance_get_export_index(
+          &instance, context, NULL,
+          "docs:adder/add@0.1.0", strlen("docs:adder/add@0.1.0"));
+  if (iface_idx == NULL) {
+    fprintf(stderr, "error: could not find export 'docs:adder/add@0.1.0'\n");
+    return 1;
+  }
+
+  wasmtime_component_export_index_t *func_idx =
+      wasmtime_component_instance_get_export_index(
+          &instance, context, iface_idx,
+          "add", strlen("add"));
+  wasmtime_component_export_index_delete(iface_idx);
+  if (func_idx == NULL) {
+    fprintf(stderr, "error: could not find function 'add'\n");
+    return 1;
+  }
+
+  wasmtime_component_func_t func;
+  bool found = wasmtime_component_instance_get_func(
+      &instance, context, func_idx, &func);
+  wasmtime_component_export_index_delete(func_idx);
+  if (!found) {
+    fprintf(stderr, "error: could not get function handle for 'add'\n");
+    return 1;
+  }
+
+  // 8. Call the function: add(x, y) -> u32
+  wasmtime_component_val_t args[2] = {
+      {.kind = WASMTIME_COMPONENT_U32, .of.u32 = x},
+      {.kind = WASMTIME_COMPONENT_U32, .of.u32 = y},
+  };
+  wasmtime_component_val_t results[1] = {0};
+
+  exit_if_error("call 'add'",
+      wasmtime_component_func_call(&func, context, args, 2, results, 1));
+
+  printf("%u + %u = %u\n", x, y, results[0].of.u32);
+
+  // 9. Cleanup
+  wasmtime_component_val_delete(&results[0]);
+  wasmtime_store_delete(store);
+  wasmtime_component_linker_delete(linker);
+  wasmtime_component_delete(component);
+  wasm_engine_delete(engine);
+
+  return 0;
+}

component-model/examples/example-c-host/justfile

@@ -0,0 +1,2 @@
+@_default:
+  echo "TODO"; exit 1