Milestone 1: basic algol 60 frontend

Analyze and build AST for most simple ALGOL 60 programs. Build such a frontend, that it can translate supported AST constructs to GENERIC. All necessary GCC infrastructure stuff is in place -- all config files, makefiles, drivers, etc. This is an example of the ALGOL program that should pass:

'begin'
  'integer' a, b;
  a := 2;
  b := 3;
  exit ((a-3)-(b-2));
'end';

Where exit is internal function used to communicate program outcome to environment.