Use Base.Math.rem_pio2_kernel for quadrant on Julia 0.7

[dpsanders]

In Julia 0.7 we can use Base.Math.rem_pio2_kernel for the quadrant calculation for trig functions, especially tan.

(Thanks to Simon Byrne for the tip.)

[lbenet]

I was checking the way mod2pi is coded in Base (because find_quadrants is essentially a discrete version of mod_pi_half function; see #117). It turns out that mod2pi(x) is a call to rem2pi(x, RoundDown); I wonder if this can be a work around that.

[dpsanders]

I tried that, but it's not good enough (or, at least, I couldn't get it to work) -- we actually need a rem_half_pi function, which is what the function in #119 provides.

[lbenet]

For floats, see what ieee754_rem_pio2; it is in Base.Math. I don't know how is rem2pi coded for BigFloats, or whether there is something like ieee754_rem_pio2 for them.

[dpsanders]

rem2pi just calls an MPFR function:

@edit rem(big"0.1", 2*BigFloat(pi), RoundNearest)

You're right that ieee754_rem_pio2 would be the correct function, but:

julia> @btime Base.Math.ieee754_rem_pio2(3.1)
  39.199 ns (2 allocations: 128 bytes)
(2, [-0.0415927, 2.43541e-18])

whereas on 0.7:

julia> @btime Base.Math.rem_pio2_kernel(3.1)
  1.648 ns (0 allocations: 0 bytes)
(2, Base.Math.DoubleFloat64(-0.04159265358979315, 2.4354103626379128e-18))

!!

[dpsanders]

Fixed by #235.

[dpsanders]

Finally rem2pi was used.