Restyling of the files/typedefs, and organization of the structures.
Moved most of the code for expression in expression.hpp/cpp
Next step is to modify the code to support modularity.
The function for creating the rule has been "extracted".
Now, expression and constraint parser have been completely separated.
In particular, the constraint parser "calls" the expression parser by using
function =prepare_expr_rule()== and by passing an appropriate builder.
Everything works as expected, all tests compile and execute correctly.
Probably, some more test is necessary: in particular we should write
a couple of tests to exercise the "failures" and "errors" of our code.
Merge branch 'master' of github.com:glipari/forts

Conflicts:
	src/syntax_trees.cpp
	tests/test_expr_tree.cpp