Add support for const blocks

[udoprog]

This PR adds support for constant blocks:

let value = const { 1 + 3 };
println!("{}", value);

Constant blocks are similarly to constant functions and declarations evaluated at compile time. They are a kind of anonymous constant declaration that can be used inline.

They have the same rules for scope evaluation as other constant eval: They can only reference and use other constant values.

There's an outstanding issue with constant evaluation that was discovered when working with this, which has been outlined in #154.