Drop DoubleFloats dependency?

[oxinabox]

DoubleFloats has a fair few dependencies.
Like SpecialFunctions, Polynomials, QuadMath etc.
(including some like Intervals.jl which trigger a fairly high number of invalidations, thus making loading this slow).
Removing it should speed up loading a fair bit.

It is only used for

TextParse.jl/src/field.jl

Lines 145 to 164 in ced8d24

	@inline function convert_to_double(f1::Int64, exp::Int)
	f = Float64(f1)
	r = f1 - Int64(f) # get the remainder
	x = Double64(f) + Double64(r)
	maxexp = 308
	minexp = -256
	if exp >= 0
	x *= pre_comp_exp_double[exp+1]
	else
	if exp < minexp # not sure why this is a good choice, but it seems to be!
	x /= pre_comp_exp_double[-minexp+1]
	x /= pre_comp_exp_double[-exp + minexp + 1]
	else
	x /= pre_comp_exp_double[-exp+1]
	end
	end
	return Float64(x)
	end

Which in turn only needs:

• Creation
• Conversion to float
• Multiply
• Divide
• ^

We could port just those functions across.
Or maybe there is another algorithm for x*10^y (integer x, y) that we can use that doesn't need it.