Deprecate `std.math : approxEqual`

The template `approxEqual` for comparing floating point numbers has
been deprecated.

Please use the template `isClose` instead, which has better default
values and is symmetric in its arguments.

To (almost) keep the current behaviour of `approxEqual(a, b)` use
`isClose(a, b, 1e-2, 1e-2)`, but we recommend to adjust the code to
make it work with `isClose(a, b)`.
