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/tests/MemoryExportsTests.cs

@@ -13,11 +13,6 @@ namespace Wasmtime.Tests
 
     public class MemoryExportsTests : IClassFixture<MemoryExportsFixture>
     {
-        public class Host : IHost
-        {
-            public Instance Instance { get; set; }
-        }
-
         public MemoryExportsTests(MemoryExportsFixture fixture)
         {
             Fixture = fixture;
@@ -44,8 +39,7 @@ namespace Wasmtime.Tests
         [Fact]
         public void ItCreatesExternsForTheMemories()
         {
-            var host = new Host();
-            using var instance = Fixture.Module.Instantiate(host);
+            using var instance = Fixture.Host.Instantiate(Fixture.Module);
 
             instance.Externs.Memories.Count.Should().Be(1);