Improve builtin Lookup

[rocky]

Lookup[] using rules is failing on examples like:

Lookup[<|a -> 1, b -> 2|>, a]
Lookup[<|a -> 1, b -> 2|>, c]
Lookup[<|a -> 1, b -> 2|>, c, 0]

This corrects that. It is not perfect: threading over lists of associations and multiple query keys are not supported. But it is better.

The operator form of Lookup is needed to support Paclet install.

[mmatera]

When a key is not in the association, WMA returns Missing[KeyAbsent, <key>]. For instance,

In[1]:= a=Association[{"F":>1,"G":>2}] ;
In[2]:=Lookup[a,"H"]
Out[2]= Missing[KeyAbsent, H]
In[3]:=Lookup[a,{"P","G"}]
Out[3]= {Missing[KeyAbsent, P], 2}

[rocky]

Thanks for checking and catching. The problem was not in the code, but instead in setting Lookup's attributes.