Support open generic metatypes

[Turnerj]

Fixes #802

Currently you can't write something like this:

RuntimeTypeModel.Default.Add(typeof(CacheEntry<>))
	.Add(1, nameof(CacheEntry<object>.Expiry))
	.Add(2, nameof(CacheEntry<object>.Value));

The problem is that the "type" is typically matched exactly however by writing such code, I'm intending it match all types that use that generic definition.

The fix is accomplished by looking up by the generic definition if it has failed to find an exact match or a contract attribute specified. If found, it generates a new metatype entry, using the fields and surrogate from the generic definition's metatype entry.

[Turnerj]

Pretty sure this part will need further refinement. I'm assigning _fields versus duplicating the field data as it should be an exact match of data - I can't think of a way this wouldn't work. Happy to instead iterate over the data and manually call AddField etc.

Also there may be other important properties to set from one MetaType instance to the other here.

[Turnerj]

I updated the code to include additional properties on MetaType that seemed relevant but I'd need your expertise to confirm my approach and whether there are any other considerations I should be aware of.

[Turnerj]

I'm kinda winging this part here based on existing code further up in the method.

[Turnerj]

This additional check seemed like the best solution to avoid triggering the next block if a metatype was generated in the new code I wrote. In my head it was either this or a goto statement.

[Turnerj]

Not sure why the build failed - as far as I can tell, all the tests passed on AppVeyor 🤷‍♂️

[Turnerj]

Hey @mgravell, if you have the time, I'd love to get your feedback on the fix - in particular, my changes in MetaType.cs are potentially controversial with how I'm initializing the new MetaType instance.

If you can't look at it at the moment because your plate is full, I totally understand!