[Issue 19732] make std.complex.expi fast.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19732

--- Comment #1 from Bastiaan Veelo <Bastiaan at Veelo.net> ---
Related documentation issues: https://issues.dlang.org/show_bug.cgi?id=19733

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