Mark associative array `byKey` and `byValue` `pure nothrow`.

This fixes part of Issue 5555.

Issue URL: https://issues.dlang.org/show_bug.cgi?id=5555
Merge pull request #838 from denis-sh/more-fixes-for-Issue-5555-byKey-and-byValue

Mark associative array `byKey` and `byValue` `pure nothrow`.