new gem: alias, tuples & static foreach

[wilzbach]

I feel that neither alias, static foreach and tuples are well represented in the tour. They are already needed in other sections (e.g. #264), so we should probably try to enhance basics/alias-strings and maybe even create a new gem. The meta programming gem also doesn't cover any of these topics.

btw if static foreach is unfamilar:

import std.meta : AliasSeq;
foreach (c, AliasSeq!(int, float))
   ...

[wilzbach]

Missing alias explanation has also been reported in #356

[wilzbach]

This issue was moved to dlang-tour/english#14