Drop to_pickable()/from_pickable() helpers

[skirpichev]

Without gmpy2:

    >>> import mpmath, pickle
    >>> pi = mpmath.pi
    >>> mpmath.mp.dps=100
    >>> a = mpmath.mpf(pi)
    >>> len(pickle.dumps(a))  # vs 147
    105

Fixes #440

• test speed

[skirpichev]

With gmpy2:

# on the master:
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import dumps' 'dumps(a)'
10000 loops, best of 10: 21.3 usec per loop
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import loads,dumps;b=dumps(a)' 'loads(b)'
10000 loops, best of 10: 24.6 usec per loop
# on this branch:
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import dumps' 'dumps(a)'
10000 loops, best of 10: 27.7 usec per loop
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import loads,dumps;b=dumps(a)' 'loads(b)'
10000 loops, best of 10: 25.1 usec per loop

Without:

# on the master:
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import dumps' 'dumps(a)'
20000 loops, best of 10: 19.8 usec per loop
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import loads,dumps;b=dumps(a)' 'loads(b)'
10000 loops, best of 10: 21.5 usec per loop
# on this branch:
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import dumps' 'dumps(a)'
20000 loops, best of 10: 14.5 usec per loop
$ python -m timeit -r10 -s 'from mpmath import mp,pi,mpf;mp.dps=100;a=mpf(pi)' \
    -s 'from pickle import loads,dumps;b=dumps(a)' 'loads(b)'
20000 loops, best of 10: 18.5 usec per loop