4.x: System.Reactive fix field names, type args, usings, operator infrastructure

[akarnokd]

• Add missing _ to private fields
• Rename internal fields to start with a capital letter
• Remove type arguments inferable from context
• Remove unnecessary this. and base. prefixes
• Remove unnecessary namespace prefixes
• Remove unused imports
• Confine certain imports to a narrower #if <label> cases
• Use IdentitySink with Amb and AmbMany
• Use object instead of Object
• Use string instead of String
• Use decimal instead of Decimal
• Use int instead of Int32
• Use long instead of Int64
• Move the common Action _onCompleted field of ActionPlan<...> into the abstract base class ActionPlan

[quinmars]

When I understand the .editorconfig right, than static readonly fields should be in PascalCase. Although it does not state if this is the case independently of the accessibility modifiers. @onovotny used PascalCase in #720.

[akarnokd]

There is a property with name Instance already.

[akarnokd]

See L24:

public static CurrentThreadScheduler Instance => _staticInstance.Value;

[quinmars]

Personally I also prefer to start private fields with an underscore else you have to become creative for backing fields. Like LazyInstance for example.

[clairernovotny]

We can always bring s_ back for static instance fields, but I thought that was nasty.

[danielcweber]

@akarnokd Sorry, I attempted to fix the conflicts and missed out on scheduler vs. Scheduler.