Define complex sqrt

[mtfishman]

Fixes #364.

@maleadt this was the most minimal implementation I could come up with.

I override two functions:

1. Base.Math.exponent(::Base.IEEEFloat) in order to avoid exception throws.
2. Base.ssqs(x::T, y::T) where T<:Real to avoid a Union type in type inference by changing this line to k = m==0 ? 0 : exponent(m).

I wonder if 2. should be fixed in Base. The Base version effectively sets k to zero(m) rather than 0 if iszero(m), however k is supposed to be Int anyway. The way that I changed it, it would probably mean that Base wouldn't need to have so many type asserts for k (there is one in Base.ssqs and one in Base.sqrt(z::Complex)).

[maleadt]

Thanks! It's too bad we have to do this; I should probably look into implementing a dynamic memory allocator.

I wonder if 2. should be fixed in Base.

I guess so, even though the exception branches don't generally hurt CPU performance it's easier on the compiler to avoid them where possible.