Ditch predicates, and let if operate on values

[xiaq]

The current boolean/predicate story is a mess:

1. Boolean outputs are on a separate channel than normal output.

2. The relationship between predicates and booleans is not straightforward.

3. If we were to add boolean operators, we perhaps need to make one set for predicates and another set for booleans.

Let's simply ditch predicates and turn them into functions that output booleans.

Now that we don't have predicates, we can rework if as well. Right now if takes a pipeline and expects it to be a predicate; since we don't have predicates, we can make if take a value instead.

The downside is that now we have a totally different if than traditional shells. But nothing is really lost: the current similarity is only syntactical and not semantic.

[xiaq]

The while control structure will also be affected by this change.