First Draft: Tuple Unpacking Syntax

[Meta]

This DIP proposes built-in tuple unpacking syntax for D. A sample 
of the proposed syntax:
```
import std.typecons : tuple;

(int a, string b) = tuple(1, "2");
assert(a == 1);
assert(b == "2");

auto (a, b) = tuple(1, "2");
static assert(is(typeof(a) == int));
static assert(is(typeof(b) == string));

auto (a, immutable b, c) = tuple(1, "2", 3.0);
static assert(is(typeof(a) == int));
static assert(is(typeof(b) == immutable string));
static assert(is(typeof(c) == double));
```

The DIP is based on Timon Gehr's old DIP for tuple syntax in D 
(https://github.com/tgehr/DIPs/blob/tuple-syntax/DIPs/DIP1xxx-tg.md), but is solely limited to support for unpacking; it is not a full tuple-syntax DIP. If the reception and general sentiment for this DIP are positive, further enhancements to add built-in tuple support to D may be proposed in the future.

Thanks to Timon and Nick Treleaven for doing the bulk of the 
implementation and conceptual work on this proposal. I mainly 
just kickstarted things and am facilitating the DIP process.

The DIP:
https://github.com/MetaLang/DIPs/blob/bf357d16b1bce65ba4ed95a08d146d4015eeb2d7/DIPs/1NNN-JH-TG-NT.md