Proposal: namespace declaration

[JipingWang]

To simplify namespace usage as the following (let's call it namespace declaration),

namespace package1 {
    // some classes for package-1
}
namespace package2 {
    // some other classes for package-2
}

to

namespace package1; 
// some classes for package-1

namespace package2; 
// some other classes for package-2

Comments:
Inspired by using declaration in C# 8.0, the proposed change saves the top level indent and a pair of braces.

[MgSam]

Duplicate of #2546, #137

[martind0]

Yes, please!

[kfimchenko]

Should be awesome in a combine with #3117

[333fred]

Closing as a duplicate of #137.