Add lowercase reactive.value, calc, effect

[wch]

This closes #814.

Many users have wondered why reactive.Value, .Calc, and .Effect are capitalized. There are technical reasons for this, but from a user perspective it makes more sense to just use lowercase names.

This PR changes those names to lowercase, and adds aliases so that the old capitalized names still work.

This PR does not change any examples because if users on a previous version of shiny were to use the example code, it wouldn't work for them, and it would be unclear to them why. In a future release, we should update the examples to use the new lowercase names.

[jcheng5]

I think we have consensus that @reactive.calc and @reactive.effect, as decorators, should be lower-cased.

I'm a little surprised to see reactive.Value() being lower-cased to reactive.value(), as that's just a plain old constructor?

[jcheng5]

And IF we do really want reactive.value to be lower-cased, I think we should still have class Value and then create a value alias (or even make a wrapper function def value(x: T) -> reactive.Value[T]) so that it still makes sense to have reactive.Value for type annotations.

[schloerke]

Oooooo. I really like the small alias functions.

• Implement lowercase names as as alias functions that return Uppercase class objects

[wch]

I think we should provide a lowercase version of value, because if we don't, I think we'll get the same questions about "why is this thing capitalized?" But I don't feel strongly about it.

I think it makes sense to keep the Value class, and then make value an alias to it.

So in short, I can roll back the Value->value changes, and then just add value as an alias. Then if we decide we don't like the alias, we can take it out, and it'll be just a one-line change.