Replace some assert:s with (new) assume:s

[janvrany]

assert:s at the beggining of methods should be replaced by assume:s, see an earlier discussion on slack:

There is assert:, and there is assume:. They are confusingly similar, just like vectors and differential forms are confusingly similar, in fact we are looking at the same mathematical phenomenon of contravariance. When I am in a function, say f(x), x>10, and I say assert(x+1 > 11), it is my (well, f's) responsibility to ensure that the assert typechecks.

But assume works "backwards", "demonic", "precondition", and "it's my caller's fault if not".

For time being, assume: can be implemented by means of assert:.

[janvrany]

See

• Save LH-Slack conversation regarding #assume: shingarov/MachineArithmetic#180
• Connect Floyd–Hoare (aka axiomatic) to game-theoretic semantics shingarov/MachineArithmetic#181