Move Wasmtime for .NET to the Wasmtime repo.

This moves the Wasmtime for .NET implementation to the Wasmtime repo.

Wasmtime for .NET is a binding of the Wasmtime API for use in .NET.

crates/misc/dotnet/README.md

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+# Wasmtime for .NET
+
+A .NET API for [Wasmtime](https://github.com/bytecodealliance/wasmtime).
+
+Wasmtime for .NET enables .NET code to instantiate WebAssembly modules and to interact with them in-process.
+
+# Getting Started
+
+## Prerequisites
+
+### .NET Core 3.0
+
+Install a [.NET Core 3.0+ SDK](https://dotnet.microsoft.com/download) for your operating system.
+
+## Introduction to Wasmtime for .NET
+
+See the [introduction to Wasmtime for .NET](https://peterhuene.github.io/wasmtime.net/articles/intro.html) for a complete walkthrough of how to use Wasmtime for .NET.
+
+# Wasmtime for .NET API documentation
+
+See the [Wasmtime for .NET API documentation](https://peterhuene.github.io/wasmtime.net/api/index.html) for documentation on using the Wasmtime for .NET types.
+
+# Running the "Hello World" Example
+
+The "hello world" example demonstrates a simple C# function being called from WebAssembly.
+
+To run the "hello world" example, follow these instructions:
+
+1. `cd examples/hello`
+2. `dotnet run`
+
+You should see a `Hello from C#, WebAssembly!` message printed.
+
+# Building Wasmtime for .NET
+
+To build Wasmtime for .NET, follow these instructions:
+
+1. `cd src`.
+2. `dotnet build`.
+
+This should produce a `Wasmtime.Dotnet.dll` assembly in the `bin/Debug/netstandard2.1` directory.
+
+To build a release version of Wasmtime for .NET, follow these instructions:
+
+1. `cd src`.
+2. `dotnet build -c Release`.
+
+This should produce a `Wasmtime.Dotnet.dll` assembly in the `bin/Release/netstandard2.1` directory.
+
+# Running the tests
+
+To run the Wasmtime for .NET unit tests, follow these instructions:
+
+1. `cd tests`.
+2. `dotnet test`.
+
+# Packing Wasmtime for .NET
+
+To create a NuGet package for Wasmtime for .NET, follow these instructions:
+
+1. `cd src`.
+2. `dotnet pack -c Release`.
+
+This should produce a `Wasmtime.<version>.nupkg` file in the `bin/Release` directory.
+
+# Implementation Status
+
+## Status
+
+| Feature                               | Status |
+|---------------------------------------|--------|
+| Wasmtime engine class                 | ✅     |
+| Wasmtime store class                  | ✅     |
+| Wasmtime module class                 | ✅     |
+| Wasmtime instance class               | 🔄     |
+| Module function imports               | ✅     |
+| Module global imports                 | ✅     |
+| Module table imports                  | ✅     |
+| Module memory imports                 | ✅     |
+| Module function exports               | ✅     |
+| Module global exports                 | ✅     |
+| Module table exports                  | ✅     |
+| Module memory exports                 | ✅     |
+| Extern instance functions             | ✅     |
+| Extern instance globals               | ✅️     |
+| Extern instance tables                | ⬜️     |
+| Extern instance memories              | ✅️     |
+| Host function import binding          | ✅     |
+| Host global import binding            | ✅ ️️    |
+| Host table import binding             | ⬜️ ️️    |
+| Host memory import binding            | ✅️ ️️    |
+| `i32` parameters and return values    | ✅     |
+| `i64` parameters and return values    | ✅     |
+| `f32` parameters and return values    | ✅     |
+| `f64` parameters and return values    | ✅     |
+| `AnyRef` parameters and return values | ⬜️     |
+| Tuple return types for host functions | ✅     |
+| Trap messages                         | ✅     |
+| Trap frames                           | ⬜️     |
+| Create a NuGet package                | ✅     |
+
+## Legend
+
+| Status | Icon |
+|-----------------|--------|
+| Not implemented | ⬜️     |
+| In progress     | 🔄     |
+| Completed       | ✅     |