Allow operations to declare a budget

One of the values I often find myself adding to the documentation of an operation is how many resources it can consume (e.g. workspace qubits, T gates). It would be nice if there was a formal way to do this, and that the claimed budget was actually verified when executing.

For example:

operation op(qs: Qubit[]) : Unit {
    budget {
        set_max_workspace(Length(qs) * 2);
        set_max_operation(CCNOT, 20);
    }
    ...
}

That being said...

This could affect the execution time of simulators.
Most programming languages don't have this sort of thing, so it'll be kind of intrinsically unfamiliar.
Not all methods have simple resource counts. E.g. probabilistic methods might have an expected value that is much lower than the worst case value.
Different simulators may see different resource counts, if operations are decomposed in different ways based on the available gate set.

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[x] I have searched both open and closed suggestions and proposals on this site and believe this is not a duplicate.
[?] I believe that the spirit of this suggestion is aligned with the design principles and general vision for Q#.

Please tick all that apply:

[x] This is not a breaking change to the Q# language design
[ ] I or my organization would be willing to help implement and/or test this

microsoft / qsharp-language

Allow operations to declare a budget #64