Review detectors for optimization

[montyly]

For example: https://github.com/crytic/slither/blob/858677464cff0f61d1553ae2ae642963ee074153/slither/detectors/functions/external_function.py#L40-L42

Could be replaced by: https://github.com/crytic/slither/blob/cf65a10a1cf59b1b523e5c510508c93b6249c979/slither/core/declarations/function.py#L881-L889

We can also write a guideline to optimize core objects usages

[0xalpharush]

@DarkaMaul This may be a simple performance win