Make SymbolRenamer public

[josefpihrt]

fixes #1156

[amsam0]

What's left to be done for this? If needed, I can probably help

[josefpihrt]

@naturecodevoid Following things would be helpful:

• Add some unit tests for SymbolRenamer (this would be the most helpful)
• Go through the API (as listed in PR's comment) and check if it fits your need.
• Download and install nuget Roslynator.Workspaces.Core from PR's artifacts and test it locally.
• Go through code and make comments

[amsam0]

The API looks like it will work for me. Ideally I would be able to use the SymbolRenamer on C# files in-memory without writing them to disk, but I don't think Roslyn's API allows for this.

I'm not the most experienced C# developer, but I'll try to add some unit tests and make a PR.