Properly represent symbolic intervals

[Stagno]

Both AST::Interval and IIR:Interval emulate symbolic markers Start and End by magic numbers (respectively 0 and 1 << 20). This lack of abstraction also led to at least one bug #1098 .

Move to a proper symbolic representation and ensure that all operations between intervals are implemented correctly and behave as they did before.

Since we re-use these intervals for the horizontal subdomains we'd also gain that we could move away from magic numbers there (also very error prone in this case).