Champion: Unmanaged constructed types (16.3, Core 3)

## Summary
[summary]: #summary

Provide a way to take the address of a generic struct provided it does not contain any "managed" (GC-tracked) types.

For example, the following generic type is "unmanaged":
```C#
public struct MyStruct where T : unmanaged
{
public T field;
}
```

## Motivation
[motivation]: #motivation

Today, users have the ability to declare structs and take the address of it (in an unsafe context) provided that it is not considered a "managed" type.
* The compiler currently reports generic structs as "managed" types, even though they are not GC-tracked

However, there is nothing in the runtime preventing a user from taking the address of a generic struct and, in certain scenarios, it may be desirable to allow this.

One such example is the `System.Runtime.Intrinsics.Vector128` type, which contains no GC tracked objects. The type is designed to be used in high-performance and generally unsafe scenarios, but there are certain operations (such as `stackalloc`, `pinning`, etc) which cannot be done in C# today.

## Design
[design]: #design

The [C# 6 spec](https://github.com/dotnet/csharplang/blob/master/spec/unsafe-code.md#pointer-types) states that an `unmanaged_type` is:

>isn't a reference_type or constructed type, and doesn't contain reference_type or constructed type fields at any level of nesting. In other words, an unmanaged_type is one of the following:

> - sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool.
> - Any enum_type.
> - Any pointer_type.
> - Any user-defined struct_type that is not a constructed type and contains fields of unmanaged_types only.

This change would remove the restriction that `unmanaged_type` cannot be a constructed type. Instead, constructed types would be unmanaged if they meet the requirements of general user-defined struct types.

## Drawbacks
[drawbacks]: #drawbacks

The compiler may need to do additional validation in order to validate that a user-defined generic struct is "ok" to use.

## Notes
[notes]: #notes

The user will still not be able to take the address of any type, which is possible to do in IL code today.