Reimplement the C# API.

This commit reimplements the C# API in terms of a Wasmtime linker.

It removes the custom binding implementation that was based on reflection in
favor of the linker's implementation.

This should make the C# API a little closer to the Rust API.

The `Engine` and `Store` types have been hidden behind the `Host` type which is
responsible for hosting WebAssembly module instances.

Documentation and tests have been updated.

crates/misc/dotnet/examples/hello/Program.cs

@@ -3,26 +3,21 @@ using Wasmtime;
 
 namespace HelloExample
 {
-    class Host : IHost
-    {
-        public Instance Instance { get; set; }
-
-        [Import("hello")]
-        public void SayHello()
-        {
-            Console.WriteLine("Hello from C#, WebAssembly!");
-        }
-    }
-
     class Program
     {
         static void Main(string[] args)
         {
-            using var engine = new Engine();
-            using var store = engine.CreateStore();
-            using var module = store.CreateModule("hello.wasm");
-            using dynamic instance = module.Instantiate(new Host());
+            using var host = new Host();
 
+            host.DefineFunction(
+                "",
+                "hello",
+                () => Console.WriteLine("Hello from C#, WebAssembly!")
+            );
+
+            using var module = host.LoadModule("hello.wasm");
+
+            using dynamic instance = host.Instantiate(module);
             instance.run();
         }
     }