Implement decimal to DECIMAL conversion on non-.NET platforms

[AArnott]

The .NET decimal type doesn't expose a proper API for extracting raw values without allocating an int[] unless you target .NET.
But .NET Standard targets can do this too by leveraging the field layout of the struct being intentionally the same between the two.

namespace Windows.Win32.Foundation
{
	/// <content>
	/// Adds implicit conversion from decimal to DECIMAL.
	/// </content>
	internal partial struct DECIMAL
	{
		public static unsafe implicit operator DECIMAL(decimal value)
		{
			// DECIMAL is layout-compatible with decimal.
			return *(DECIMAL*)&value;
		}
	}
}

Also, the converter that is emitted is buggy:

public static implicit operator decimal(DECIMAL value)
{
	return new decimal(
(int)value.Anonymous2.Anonymous.Lo32, (int)value.Anonymous2.Anonymous.Mid32, (int)value.Hi32, value.Anonymous1.Anonymous.sign == 0x80, value.Anonymous1.Anonymous.scale);
}

This throws OverflowException as it casts uint fields to int values without unchecked.