D semantics, shared as a heap storage specifier

[Etienne Cimon via Digitalmars-d]

This is the only issue preventing a truly thread-local GC for better 
multi-core scalability for D applications. From: 
https://github.com/D-Programming-Language/druntime/pull/1057#issuecomment-65904128

The best way to achieve a thread-local GC would be to improve and 
enforce `shared`-correctness in Phobos/druntime (at first). We need to 
start considering `shared` as a heap storage attribute as well, for 
consistency. An optional compiler warning (through a flag) would be a 
start.

If even a 30% speedup is possible down the line, it's worth it. The more 
threads, the more improvements.

There's also some new opportunities with this. Here's an example that 
involves TLS data to influence the behavior of shared objects, without 
using a global `T[Thread]` hashmap.

```D
shared class A {
	private bool m_init; // different on every thread
	
	public shared:
	AA m_impl;
	
	synchronized void init() {
		if (!m_init)
			m_impl.add(Thread.getThis());
	}
	
	...
	
	
}
```