Compile-time `const` values

[Wodann]

Similar to Rust's const we want to introduce compile-time constants using the const keyword. E.g.:

const UPPER_CAST_CONST: usize = 5;

[baszalmstra]

Some thoughts on the implementation:

• We can create IR constants that are compiled into the source that uses it
• We can create a function (or a memory location really) that holds the value of the constant (which can then be patched at will)
• Type changes also require code changes so that shouldn't be an issue.
• With the not hotreloading case, we simply create an IR constant.

[baszalmstra]

A maybe more interesting topic is what we allow as the value of a constant? We don't have anything like constexpr yet.