Improve bessel_j in transition region

[fredrik-johansson]

• Add acb_hypgeom_bessel_j_deriv_bound
• Mitigate cancellation for $J_n(z)$ in the transition region and on wide intervals, by using increased precision + derivative bounds.
• Essentially only implemented for small integer $\nu$, though evaluation should also be improved in some other cases at least when both $\nu$ and $z$ exact.

Some numerical examples:

>>> sum(CC.bessel_j(0, k) for k in range(100))
[1.41921 +/- 3.56e-6]                # before
[1.419207859380 +/- 2.35e-13]        # after

>>> w = CC.exp_pi_i(QQ(1)/100); sum(CC.bessel_j(0, w*k) for k in range(101))
([0.7803 +/- 4.35e-5] + [0.6276 +/- 6.52e-5]*I)                   # before
([0.78030446659 +/- 2.44e-12] + [0.62756344686 +/- 5.49e-12]*I)   # after

>>> CC.bessel_j(1, "10 +/- 5")
[+/- 1.75e+5]    # before
[+/- 0.370]      # after

>>> CC.bessel_j(1, "100 +/- 5")
[+/- 0.361]      # before
[+/- 0.0848]     # after

>>> CC.bessel_j(1, "10 +/- 0.01")
[+/- 7.77]          # before
[0.04 +/- 6.00e-3]  # after

>>> CC.bessel_j(0, "10 +/- 100")
FlintUnableError: ...   # before
[+/- 1.01]              # after

>>> CC.bessel_j(0, CC("10 +/- 100") * CC.i())
FlintUnableError: ...   # before
[+/- 5.93e+47]          # after

>>> CC.bessel_j(0.5+0.75j, 26)
([0.20772 +/- 3.94e-6] + [-0.14801 +/- 2.12e-6]*I)                        # before
([0.207717454673942 +/- 6.42e-16] + [-0.148010904937155 +/- 5.17e-16]*I)  # after

Added an example integral:

# before
fredrik@mordor:~/src/flint$ build/examples/integrals -i 38 -verbose
I38 = int_0^{1000} J_0(x) J_1(x) J_2(x) dx ...
depth 6/128, eval 4126/68096, 105 leaf intervals
cpu/wall(s): 0.168 0.167
I38 = [0.057674 +/- 1.03e-7]

# after
fredrik@mordor:~/src/flint$ build/examples/integrals -i 38 -verbose
I38 = int_0^{1000} J_0(x) J_1(x) J_2(x) dx ...
depth 5/128, eval 1688/68096, 16 leaf intervals
cpu/wall(s): 0.058 0.058
I38 = [0.057673961810863 +/- 1.35e-16]

This wants a lot of fine-tuning and something better for non-integer $\nu$, but one has to start somewhere...