`import std.experimental.all` as a global convenience import

$(MREF std,experimental,all) allows convenient use of all Phobos modules
with one import:

---
import std.experimental.all;
void main()
{
    10.iota.map!log.sum.writeln;
}
---

For short scripts a lot of imports are often needed to get all the
modules from the standard library.
With this release it's possible to use `import std.experimental.all` for importing the entire
standard library at once. This can be used for fast prototyping or REPLs:

---
import std.experimental.all;
void main()
{
    6.iota
      .filter!(a => a % 2) // 0 2 4
      .map!(a => a * 2) // 0 4 8
      .tee!writeln
      .sum
      .writefln!"Sum: %d"; // 18
}
---

As before, symbol conflicts will only arise if a symbol with collisions is used.
In this case, $(LINK2 $(ROOT)spec/module.html#static_imports, static imports) or
$(LINK2 $(ROOT)spec/module.html#renamed_imports, renamed imports) can be used
to uniquely select a specific symbol.

The baseline cost for `import std.experimental.all`
is less than half a second (varying from system to system) and
work is in progress to reduce this overhead even further.
