Also update a couple docs to reflect more modern information. Docs are
very out of date at this point, and a review pass should probably be
done.

Remove the AnalyzerManager singleton to prevent storing analyzer execution state across compilations

Now we throw away all the analyzer execution state once the CompilationWithAnalyzers goes out of scope.

Ignore tuple name and dynamicness differences in computing constant value for as operator on nullable tuple (#19697)

* Update toolset compilers to 2.3.0-beta2-61716-09. Use LangVersion=7.1
* Revert LangVersion for VB. Follow-up issue #19610

* Fixes crash in `is` operator used with tuples
Fixes #19310

* Matching an expression of type `byte?` with a pattern of type `int` should not be allowed
* Add documentation for the breaking change matching `int?` against the pattern `long x`
Fixes #19151

* Unify subsumption implementations
Fixes #19273

* Greatly improved code gen for value type pattern match
Fixes #16129
Fixes #18944
Fixes #14252

* Add test confirming that #19122 is fixed.
Fixes #19122

The intent of CONTRIBUTING.md is to provide guidelines for contributing to a
repository.  Ours was a mix of guidelines, best practices and developer
workflow.  The other aspects are already covered in our other contributing
documents.

Changing CONTRIBUTING.md to be guideline focused.

* Ref assemblies make a best effort to emit with determinism
* Fixing CopyRefAssembly message output
* CopyRefAssembly logs when input isn't ref assembly
* CopyRefAssembly does assembly redirect.
* Compact error codes for C# 7.1

The example ruleset provided contains some syntax errors, so copy/pasting wholesale somewhere can result in build errors when including it.

There was also a minor typo I found later in the file while reading through.

There is no good method today for executing windows commands with
dynamically built argument lists in Powershell.  This change attempts
to address that by introducing Exec-Command which uses the .NET
Process API directly.  This makes it straight forward to build windows
commands that rely on windows style parsing and output