Closes#373
This forbids expressions such as `a and b or c`, avoiding the need to set an
implicit priority between `and`, `or` and `xor`, which I find error-prone.
Instead, when that appears, a message asking for explicit parentheses will be
shown to the user.
Implementation note: since that would be extremely tedious to do in the parser
directly, the parser is set to allow right-associativity without discrimination
for the logical operators, and the check is done during desugaring. This
required to explicit parentheses in the surface AST to discriminate the case
where the priority was explicit.
Command used: `sed -i 's/\([-+*/><=]=\?\)[.$@^€$]/\1/g' **/*/*.catala_*`
The overload test, of course, is kept unchanged and ensures that explicit
operators still work.
