[nem-en] Warning levels

[Kamil Skalski]

We should probably add the "warning level" feature to ncc. It will allow us
easier organization of warnings (the new kind of warnings, often unwanted,
are appearing more often). The difficult decision is what warning should go
to which level and what should be the default level.

Maybe we should also have numbers of errors and warnings? This would have
two large advantages:
- easier handling of flags disabling / enabling warnings - the code deciding
whether to emit message or not would be in Message class, not splitted
through code
- we could make some kind of specification, where warnings would be listed
(and localized :P), so we could head for better testing of compiler (each
warning and error has its own test). We could use C# error list. For sure
some analysis of this list would help us specifying and testing compiler.
- probably we could also unify errors (at least those nonfatal) with
warnings and later (in Message) decide if it is a warning or error
(depending on wlevel)

BTW. we should make some internal (or semiinternal) list of differences
between
C# and Nemerle regarding some minor issues with OO features etc.
(for example C# allows to have `class A{ public prop : int { get {..}} }
class B : A { public prop : int { get {} }}'  and implicitly add `new'
modifier, which is an error in Nemerle and IMHO should stay so).
Most of them are not good to be reported as bug in Mantis, but it would be
good to add some noted to those entries.

Kamil Skalski