Enable Python 3.8 on Appveyer and also for manylinux builds

Since 3.8 is not available on Appveyor yet, use a script to install it.

+ Test pure versions and docs in TravisCI.

Fix C compilation warning

PRIx64 is defined in <inttypes.h>, not <stdint.h>.

I hate C.

gcc 8.3.0 emits this during python setup.py build:

persistent/cPersistence.c: In function ‘Per_repr’:
persistent/cPersistence.c:1481:44: warning: format ‘%llx’ expects argument of type ‘long long unsigned int’, but argument 4 has type ‘uint64_t’ {aka ‘long unsigned int’} [-Wformat=]
         snprintf(buf, sizeof(buf) - 1, "%llx", oid_value);
                                         ~~~^   ~~~~~~~~~
                                         %lx

<stdint.h> defines standard macros such as PRId64, PRIu64 and PRIx64 for
printing {u,}int64_t values, so let's use them.

Use Py_ssize_t for length arguments of PyArg_ParseTuple

This API was introduced in Python 2.5 and the old one (using ints) has
been deprecated since Python 3.8.

See #108.

This API was introduced in Python 2.5 and the old one (using ints) has
been deprecated since Python 3.8.

Fixes #108.

Require CFFI on CPython for pure-Python operation.

Fixes #77

Allow sweeping cache without cache_size and with cache_size_bytes only.

This reverts commit e6d183b1.

Fully test the C implementation of the PickleCache

Fix discrepancies between it and the Python implementation:

  - The C implementation now raises ``ValueError`` instead of
    ``AssertionError`` for certain types of bad inputs.
  - The Python implementation uses the C wording for error messages.
  - The C implementation properly implements ``IPickleCache``; methods
    unique to the Python implementation were moved to
    ``IExtendedPickleCache``.
  - The Python implementation raises ``AttributeError`` if a
    persistent class doesn't have a ``p_jar`` attribute.

Fixes #102

Exclude the 'terryfy' directory from wheels and sdists

Always format 8-byte oids as ints in hexadecimal.

Fixes #99