remap offsets from bytes to chars during source map conversion

[bohendo]

Remaps byte offsets to char offsets in the _convert_source_mapping method of the Source class.

This method now:

• uses utf-8 encoding to convert the source code string to a bytes array
• applies the byte offsets
• decodes back to a normal string
• uses the length of the resulting string to determine new char-denominated offsets

Assumptions:

• this method is only ever used to ingest output from the solc compiler, which infamously outputs byte-denominated src map offsets.
• other platforms (eg foundry, hardhat) already provide char-denominated src map offsets

Problem: if f not in sourceUnits then we don't have reliable access to the source filename or code so we can't use the aforementioned method to convert to byte offsets. It's not clear to me when or why this logic branch would be taken, however.

[github-actions]

[blackfmt] _{reported by reviewdog 🐶}

Suggested change

	def _convert_source_mapping(
	offset: str, compilation_unit: "SlitherCompilationUnit"
	) -> Source: # pylint: disable=too-many-locals
	) -> Source:
	def _convert_source_mapping(offset: str, compilation_unit: "SlitherCompilationUnit") -> Source:

[bohendo]

This PR demonstrates a deep misunderstanding of the underlying problem