Fix parser associativity bug

[denismerigoux]

The problem

The following Catala program:

declaration scope A:
  output x content money

scope A:
  definition x equals $4 -$ $2 -$ $2

Outputs x = $4 instead of x = $0. The associativity of the $- operator is wrong, is associates to the right but should be left. The problem is likely to be the same for all minus operators.

The fix

The parser should be fixed to respect the usual associativity rules. This is a high-priority bug, since it is likely to impact users in very deceitful ways.