add overload of ::new for placement support

[cfmkla]

This adds an overload of ::new that matches the signature required for placement new. For additional background, here's what cppreference has to say about placement new.

When a programmer calls new Foo the existing overload of new is called which uses malloc to provide the required amount of memory. To use placement, the programmer calls new(ptr) Foo where ptr is a correctly aligned pointer to sufficient unused memory to hold an instance of Foo. The overload added in this PR is called which returns the passed in pointer to be used as the address of the newly created object.

To destroy an object allocated with placement new call its destructor explicitly: fooPtr->~Foo(). There is no corresponding "placement delete".

The following is a trivial example of placement new in action:

void *ptr = malloc(sizeof(Foo))
Foo *fooPtr = new(ptr) Foo;
fooPtr->~Foo();
free(ptr);

[aentinger]

Placement new was already proposed in #33 and has just been merged.