Number of ulps in :accurate mode

[dpsanders]

Currently in the :accurate directed rounding mode, we round the results of operations outward by adding or subtracting 1 ulp.

However, different functions at different precisions may have different numbers of ulps that they differ by if they are not correctly rounded.

We should take account of this using a function called e.g. ulp:

ulp(::typeof(sin), x::Float64) = 1
ulp(::typeof(tan), x::Float32) = 2

and call this function to determine how many ulps to round outwards by.