Replaced `approxEqual` by `isClose` in std.math.

The template `approxEqual` for comparing floating point numbers has
been replaced by the template `isClose`, which has better default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)`.

In one of the next releases `approxEqual` will be deprecated and
eventually removed.
