langref: document destructuring syntax

[perillo]

Support for destructuring/unpacking of tuples, arrays and vectors was added in 88f5315. Commit aec821c updated the grammar in langref.html.

What's missing is documenting the new syntax, probably in a new section?

Thanks.

[perillo]

@expikr, you can find same examples in https://github.com/ziglang/zig/blob/master/test/behavior/destructure.zig.

[perillo]

@expikr, a solution is to update the https://ziglang.org/documentation/0.11.0/#Compile-Time-Expressions section, adding examples of variable declaration, both the classic and the new destructuring syntax.

There are several examples in the Language Reference (I counted 4), but none in the compile-time-expressions section.