Also, improve test coverage for FileStream.SetLength

* Support IList<string>

* Revert API change

* Feedback

* Use ReadOnlySpan

* Feedback

* Fix comment

* address feedback

* Add test for List<string> optimization

* Remove StartIndex and Count in core

* Update src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>

- Fix issue where method token referred to derived type but method was actually defined on base type
  - Previously the methods did not carry enough state to determine when to emit the owner type of the method, and which exact type was the owning type. The new logic computes it from the token in the presence of a proper instantiation context, which allows for correct operation.

- Fix issue where constrained dispatch on a method of a valuetype would not put in the correct owner type
  - Issue fixed by determining if the token resolves to a method on the same type as the eventual target method, or if it needs to have a specific owning type specified

In general these issues where both caused by confusion around exactly the correct owning type, and it turned out that computation could not be computed within the signature emitter code, but instead needed to be computed in the JIT at point of use. Fortunately, we had a structure `MethodWithToken` that is used in these (and only these situations). Finally, I also made a pass through the emitter and related logic to remove various band-aids that had built up over the last few years to make all the tests and applications pass. I believe that the new logic should be correct in the general case.

Bonus tweak...  Use parallelism when compiling the framework composite images with crossgen2, and fix bug in composite image generation where mangled symbol names might conflict.

Fixes #43466 and fixes #43467

* Remove unsafe code from String.Join

- Use a span for the separator to share the same code across a char separator and string separator, rather than using pointers.
- Consolidate argument validation for the value/startIndex/count overloads into one shared helper
- Change indexing to use standard 0 to values.Length for loop to eliminate bounds checking on span accesses

* Update src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

* Remove size_t from managed Brotli code

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Co-authored-by: Stephen Toub <stoub@microsoft.com>