Force assert() to accept only booleans

Currently assert() will accept non-boolean values, and tests them for truthyness/falsiness. Sometimes this is desirable, but it may permit problems such as mistakenly testing an object, or a function itself (rather than its return value), and so on.

It would be better if assert() only accepted boolean values, and situations where the author wants to test a non-boolean value should explicitly declare that by using assert.truthy() and/or assert.falsy() helper functions.