Strict left-to-right expression evaluation - no operator precedence!

Operator precedence has been removed from the language. Expressions
are now evaluated strictly from left to right, and there are no
right-associative operators.

The old operator precedence logic is still there, but it's only used
for issuing warnings whenever an expression would have been evaluated
differently by previous versions of EEL. These warnings may (and
should!) be eliminated by rearranging the expression in a way that
evaluates the same with or without operator precedence.

The warnings can be disabled for the current source file using the
directive 'noprecedence'. As the expression evaluation warnings are
removed later, this directive will be deprecated and generate
warnings, and will eventually be removed again.

Updated all test suite and EELBox tests/examples/programs.

Added some expressions designed to break with operator precedence,
to arithmetics.eel.

Added mouse click exit to gltest.eel.

Closes #7.