[nem-en] macro to define a new kind of member

[Kamil Dworakowski]

I am working on the AOP macros and want it to have a nice syntax. For 
that purpose I would like to have a possibility to define syntax of a 
new member (this will allow for a better implementation of NemerleUnit 
also) by means of a macro. Needed to define advice and pointcut.

I have been able to modify MainParser.ParseClassMember so as it would 
try to parse top extension targeted at a class and add it to the 
modifiers of the enclosing type. I hope I have not broken anything due 
to ignorance. Don't worry it is careful about nested types, though.

I have committed it to the repository but am not fully satisfied. I 
would also like to pass modifiers as well as custom attributes as 
parameters to that macro. I was thinking of adding a new macro target, 
namely, WithingClass. Such target would (enforce/generate if not 
present) two additional parameters for attrs. Someone has another idea?

Kamil Dworakowski

PS. Compilers's code is cleverly structured, I've enjoyed working on it.