Implement toString that supports %f.
Add deprecation date.

Improve documentation for new toString.
Add TODO to make toString() pure @safe nothrow

I tried each of those qualifiers but none of them compiled due to the
way std.format works right now. So leave it for the future.
Merge pull request #1516 from quickfur/complex_format

Fix issue 10881: support writefln("%f") of complex numbers