Add assert_comptime builtin to panic if a value is not compile-time known

[jfecher]

Problem

After #2178 is merged it will be more difficult for users to track whether a value can be used as a for loop bound or if it will be efficient to use as an array index or not.

Happy Case

We should add an assert_comptime function for debugging which will panic before the loop unrolling step in SSA if a value is not known by that time. This time was chosen because the loop unrolling step is the only step (currently) that will error if a value (loop bounds) are not known at compile-time.

Alternatives Considered

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Additional Context

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