Add cascaded method invocation feature

[xshaheen]

Summary

Add method cascades. Method cascades provide a syntactic sugar for situations where the object of a method invocation might otherwise have to be repeated.

Motivation

Make it easer to write fluent API.
With cascades, we will gain fluent API on the .NET base library.

Syntax

The use of .. is to cascaded method invocation operation. The .. syntax invokes a method (field or property) but discards the result, and returns the original object instead.

List<int> list = new();

list
..Add(1)
..Add(2)
..Add(3);

String s = (new StringBuilder()
..Append("Hello")
..Append(" ")
..Append("Word")
).ToString();

[HaloFour]

#781

[Kimi-Arthur]

This is quite different from what's proposed in #781 , which messes with typing and require void for the original method return type.

[HaloFour]

Those would be design and implementation details for effectively the same feature request.

[Kimi-Arthur]

MgSam's objection won't apply for the design in this proposal. Syntax is key here as it's for language design.

[HaloFour]

IMO that still doesn't necessitate a completely separate discussion. The idea is the same.