Change fixity of `if_then_else_` in `Class.ToBool` to 0

[carlostome]

This is to match the stdlib and Decidable

[WhatisRT]

This seems reasonable. @omelkonian I think you might have written this, do you know why it's currently at -10?

[omelkonian]

Don't remember, but even if there was some reason, it wouldn't be reason enough to deviate from the standard precedence of such a widely used syntactic construct.

👍 for changing to 0