* Make DER reader into a context manager

* Added another test case

* flake8

* Remove non-test dependencies on asn1crypto.

cryptography.io actually contains two OpenSSL bindings right now, the
expected cffi one, and an optional one hidden in asn1crypto. asn1crypto
contains a lot of things that cryptography.io doesn't use, including a
BER parser and a hand-rolled and not constant-time EC implementation.

Instead, check in a much small DER-only parser in cryptography/hazmat. A
quick benchmark suggests this parser is also faster than asn1crypto:

  from __future__ import absolute_import, division, print_function
  import timeit

  print(timeit.timeit(
      "decode_dss_signature(sig)",
      setup=r"""
  from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
  sig=b"\x30\x2d\x02\x15\x00\xb5\xaf\x30\x78\x67\xfb\x8b\x54\x39\x00\x13\xcc\x67\x02\x0d\xdf\x1f\x2c\x0b\x81\x02\x14\x62\x0d\x3b\x22\xab\x50\x31\x44\x0c\x3e\x35\xea\xb6\xf4\x81\x29\x8f\x9e\x9f\x08"
  """,
      number=10000))

Python 2.7:
  asn1crypto: 0.25
  _der.py: 0.098

Python 3.5:
  asn1crypto: 0.17
  _der.py: 0.10

* Remove test dependencies on asn1crypto.

The remaining use of asn1crypto was some sanity-checking of
Certificates. Add a minimal X.509 parser to extract the relevant fields.

* Add a read_single_element helper function.

The outermost read is a little tedious.

* Address flake8 warnings

* Fix test for long-form vs short-form lengths.

Testing a zero length trips both this check and the non-minimal long
form check. Use a one-byte length to cover the missing branch.

* Remove support for negative integers.

These never come up in valid signatures. Note, however, this does
change public API.

* Update src/cryptography/hazmat/primitives/asymmetric/utils.py

Co-Authored-By: Alex Gaynor <alex.gaynor@gmail.com>

* Review comments

* Avoid hardcoding the serialization of NULL in decode_asn1.py too.