Expand `reduceBool` and add `reduceSlice`

[Gusarich]

resolves #195

I didn't cover that with new tests yet, going to do it soon

[Gusarich]

Current implementation doesn't actually work and throws errors on compiling new test cases. I'll investigate the problem and try to fix it.

[anton-trunov]

The issue here is that constant evaluation is performed on a non-typechecked AST. So when you call getExpType to dispatch on the type of == / != operators, there is no actual type information. The proper fix here would be to refactor the compiler to perform constant evaluation after typechecking (and to actually do this refactoring we need a new type describing typed ASTs).

[anton-trunov]

Looks like this PR should be closed now? Because the architecture of the constant evaluator has changed a lot in #432. Of course, we can still improve a lot of things in the constant evaluator, adding support for slices would be especially nice.

[Gusarich]

yes, right