std.essentials

[Ogi]

DMD 2.086.0 introduced `import std`. This is very convenient for 
scripts and small apps. The downsides are compile time, the 
resulting file size, and pollution of global namespace.

When writing scripts, the file size is no concern, you just want 
to get things done as fast as possible. Globally import symbols 
can give you some fun debugging session (Oh, you forgot to define 
`name`? It’s time for you to learn about `std.compiler : name`, 
baby!). But the real dealbreaker is compile time. These extra 
hundreds of milliseconds starts to matter when you’re prototyping 
your script and recompile it every few seconds. With `import.std` 
you lose more time than you save by not importing things manually.

But if you think about it, there’s only a small portion of Phobos 
that you want to have in the most cases. So I have an idea of 
some `std.essentials` module that would publicly import the most 
commonly used Phobos modules like `std.traits`, `std.range`, 
`std.algorithm`, `std.conv` etc.

To try it out, I created a file that looks like this:

```
module essentials;

public {
	import std.traits;
	import std.range;
	import std.algorithm;
	import std.array;
	import std.string;
	import std.typecons;
	import std.stdio;
	import std.conv;
	import std.format;
	import std.exception;
}
```
I’ve tried it with a hello world (only uses `writeln`) and a more 
complex app that makes use of `std.algorithm` and such both at 
runtime and at compile time. Results:

Compilation time, ms (measured with Windows PowerShell 
`Measure-Command`):

|  | manual imports | `import essentials;` | `import std;` |
|-|-|-|-|
| hello world | 176 | 233 | 610 |
| test app | 411 | 410 | 738 |

File size, KB:

|  | manual imports | `import essentials;` | `import std;` |
|-|-|-|-|
| hello world | 262 | 262| 637 |
| test app | 327 | 327 | 663 |

I didn’t do more rigorous tests but this already looks very 
promising.

This also deals with namespace pollution, we are only importing 
the most common and well-known stuff from Phobos.