abstract layouts

[Ty Young]

Hi,


When decorating a layout with attributes, there is no way to declare 
that some attributes need to be filled in by an API user. In order to 
fix this, I'd like to introduce some simple changes in order to support 
that[1]:


1. withAttribute no longer accepts a null value as an attribute value.

2. Addition of an asAbstract method[2], which takes in a var args of 
attributes that need to be filled in(indicated by a null value).

3. Addition of an isAbstract method[3], which returns true if any 
attribute has a value of null.

4. Addition of an unfilledAttributes method[4] which returns a list of 
all attributes that have a null attribute value and need filled in.


There are some minor code style consistency mistakes with the asAbstract 
method and the docs could be improved, but this is more about just 
putting the idea out there.


[1] https://github.com/BlueGoliath/panama-foreign/tree/abstract_layouts


[2] 
https://github.com/BlueGoliath/panama-foreign/blob/a1c81e0b2b929cc3e1738c31acc97830294e0346/src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryLayout.java#L251


[3] 
https://github.com/BlueGoliath/panama-foreign/blob/a1c81e0b2b929cc3e1738c31acc97830294e0346/src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryLayout.java#L610


[4] 
https://github.com/BlueGoliath/panama-foreign/blob/a1c81e0b2b929cc3e1738c31acc97830294e0346/src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryLayout.java#L342