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/misc/dotnet/src/TrapException.cs

@@ -29,9 +29,11 @@ namespace Wasmtime
                 Interop.wasm_trap_message(trap, out var bytes);
                 var byteSpan = new ReadOnlySpan<byte>(bytes.data, checked((int)bytes.size));
 
-                int indexOfNull = byteSpan.IndexOf((byte)0);
+                int indexOfNull = byteSpan.LastIndexOf((byte)0);
                 if (indexOfNull != -1)
+                {
                     byteSpan = byteSpan.Slice(0, indexOfNull);
+                }
 
                 var message = Encoding.UTF8.GetString(byteSpan);
                 Interop.wasm_byte_vec_delete(ref bytes);