Give operators precedence using the shunting-yard algorithm

Give operators precedence using the shunting-yard algorithm

I adopted the C language operator precedence, with all the controversy
it may cause with regards to the bitshift operators. Please note that
I even kept the precedence of exclusive OR over inclusive OR (for
example Perl treats them both with same precedence).

Note that two pseudo-operators are added:

 - ABO_LPAREN: used only during parsing
 - ABO_RPAREN: never even exists as an atom; it is used to stop stack
               reduction at the nearest left parenthesis

I could use ABO_OR instead of ABO_RPAREN, but it would make the code
hard to maintain if I want to add another operator with lower precedence
than OR (e.g. the ?: operator).

I'm using incomplete atom_bin structures for the operator stack. This
means that I have to check whether atom1 and atom2 exists when I free
the atom.