Use parity formula for besseli

See
https://functions.wolfram.com/Bessel-TypeFunctions/BesselI/04/02/01/0002/

Closes #877

mpmath/functions/bessel.py

@@ -82,6 +82,8 @@ def besselj(ctx, n, z, derivative=0, **kwargs):
 def besseli(ctx, n, z, derivative=0, **kwargs):
     n = ctx.convert(n)
     z = ctx.convert(z)
+    if ctx.nint(n) and n < 0:
+        return ctx.besseli(-n, z, derivative, **kwargs)
     if not z:
         if derivative:
             raise ValueError

mpmath/tests/test_functions2.py

@@ -91,6 +91,12 @@ def test_bessel():
         mp.dps = n
         assert besseli(91.5,24.7708).ae("4.00830632138673963619656140653537080438462342928377020695738635559218797348548092636896796324190271316137982810144874264e-41")
 
+def test_issue_877():
+    mp.dps = 64
+    r = besseli(-127, 2)
+    assert besseli(127, 2) == r
+    assert r.ae("3.345358761443415013354345973251886375421555647081543375756063117036e-214")
+
 def test_bessel_zeros():
     assert besseljzero(0,1).ae(2.40482555769577276869)
     assert besseljzero(2,1).ae(5.1356223018406825563)