Refactor source mapping + add new source code APIs

[montyly]

• core.source_mapping.SourceMapping is now a class (fix Rewrite source mapping class #695)
• Add slither.offset_to_objects to retrieve the object on a given offset (WIP)
• Add slither.utils.source_mapping APIs to allow retrieving definition/implementations/references informations from an offset (WIP)

[lgtm-com]

This pull request introduces 1 alert and fixes 13 when merging 288deec into cd134c4 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 13 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 13 when merging 58ae66a into be60c73 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 13 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 13 when merging 5b7a458 into da326aa - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 13 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging a7d8a42 into 65383b9 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging aed9604 into 5abe863 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging 8da99e6 into 5abe863 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging d18a3da into 0342506 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging a5489f5 into 0342506 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 15 when merging 8eb481b into 018d0df - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 15 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 16 when merging 091fe7a into 3a934a9 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 16 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert and fixes 16 when merging 48b5202 into 3a934a9 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 16 for Missing call to `__init__` during object initialization

[montyly]

This PR also adds a new flag, --change-line-prefix, to change the prefix used to display source code (default # - file.sol#1). This can be useful to output to vscode, or other IDE