Breaking Change: Remove `=` as a comparison and `not` as a keyword

[Ashvin-Ranjan]

This will create a lot of confusion once N released

= as a comparison

= is usually used for assigning values and it will be very confusing to the people using N if they see = being used as a comparison, and it might cause syntactical ambiguity somewhere down the line, this is not even very hard to replace as we already accept == as a comparison

not as a keyword

If we are going to keep not as a keyword we should also add and and or for consistency, otherwise there is no reason for not to be in N at all given that we have ~

Notifying @SheepTester on this.