pyqir-evaluator should gracefully fail when bitcode contains unknown external functions

[LaurentAjdnik]

Describe the bug

When an LLVM file contains declarations of external functions that are unknown, the evaluator crashes without displaying an error message or raising an Exception.

Issue related to #121.

To Reproduce

Use an LLVM file that contains unknown external functions, such as:

%String = type opaque

declare %String* @__quantum__rt__bool_to_string(i1)

define void @main() #1 {
entry:
  call %String* @__quantum__rt__bool_to_string(i1 1)
  ret void
}

attributes #1 = { "EntryPoint" }

This evaluation does not reach the end and throws no Exception:

from pyqir.evaluator import GateLogger, NonadaptiveEvaluator

print("Start")

try:
    NonadaptiveEvaluator().eval("CrashMe.ll", GateLogger())
except Exception as e:
    print("An exception was raised.")

print("End")

Expected behavior

The evaluator should either:

• Handle them in a transparent manner (if possible)
• Display an error message ("Unsupported function...")
• Raise an exception

[idavis]

I think the bind function in qirlib/src/evaluation/runtime.rs needs to be updated to something like

    fn bind(module: &Module<'ctx>, ee: &ExecutionEngine<'ctx>) -> Result<(), String> {
        let unknown_functions = module_functions(module).filter(|f| {
            is_extern(*f) && !is_supported_qis_instrinsic(*f) && !is_supported_rt_instrinsic(*f)
        });
        // if collection isn't empty, bubble up error about unsupported functions

module_functions is defined in qirlib/src/evaluation/jit.rs but the other functions don't exist.