It is now split into arithmetic.jl, elementary.jl and minmax.jl. The
corresponding tests have also been split into three files.

The file arithmethic.jl contains +, -, *, / and ^ as well as sign,
abs, real, imag and conj.

The file elementary.jl contains elementary functions such as exp, sin
and cos.

The file minmax.jl contains min, max, minmax, minimum, maximum and
extrema.

Adds the following new methods:
- Multi argument versions of + and *
- fma and muladd
- cube
- signbit

It also adds a special case for binary operations with irrationals,
reducing the number of allocations.

Adds multi argument versions for min and max.