Review detectors for optimization

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

Compare slither/detectors/functions/external_function.py with slither/core/declarations/function.py at the referenced lines. Review how detector code uses core objects and identify other optimization opportunities across the detectors. Done means the relevant usages have been reviewed and the resulting optimization guideline is documented.

Written by the indexing model from the issue text.