[nem-en] Typing a expression in context of method

[vc]

> Maybe just replace mb.Body with FunBody.Abstract(), call Typer(mb) and
> replace it back again?
> 
> I agree it's kind of ugly, but is easy and should do the trick.

Yes, very ugly.

Maybe pick out RunTyper from constructor and add to RunTyper parameter
through which pass expression for typing? And so as to RunTyper return TExpr
Instead of replacing corresponding MethtBuilder state (convert logic of
Typer to functional manner)? At now logic of Typer wery hard to understand. 

Add will be very nice if Typer will get reference to Messenger parameter
instead of get it from solver (in Integration we type methods in lazy manner
and we need intercept messages from typer).

Vlad