Asn1crypto

- Added `core.load()` for loading standard, universal types without knowing
the spec beforehand
- Added a `strict` keyword arg to the various `load()` methods and functions in
`core` that checks for trailing data and raises a `ValueError` when found
- Added `asn1crypto.parser` submodule with `emit()` and `parse()` functions for
low-level integration
- Added `asn1crypto.version` for version introspection without side-effects
- Added `algos.DSASignature`
- Fixed a bug with the `_header` attribute of explicitly-tagged values only
containing the explicit tag header instead of both the explicit tag header
and the encapsulated value header

- Added support for year 0
- Added the OID for unique identifier to `x509.NameType`
- Fixed a bug creating the native representation of a `core.BitString` with
leading null bytes
- Added a `.cast()` method to allow converting between different
representations of the same data, e.g. `core.BitString` and
`core.OctetBitString`

- Force `algos.DigestAlgorithm` to encoding `parameters` as `Null` when the
`algorithm` is `sha1`, `sha224`, `sha256`, `sha384` or `sha512` per RFC 4055
- Resolved an issue where a BER-encoded indefinite-length value could not be
properly parsed when embedded inside of a `core.Sequence` or `core.Set`
- Fix `x509.Name.build()` to properly handle dotted OID type values
- `core.Choice` can now be constructed from a single-element `dict` or a
two-element `tuple` to allow for better usability when constructing values
from native Python values
- All `core` objects can now be passed to `print()` with an exception being
raised

- Don't fail importing if `ctypes` or `_ctypes` is not available

- `core.Sequence` will now raise an exception when an unknown field is provided
- Prevent `UnicodeDecodeError` on Python 2 when calling
`core.OctetString.debug()`
- Corrected the default value for the `hash_algorithm` field of
`tsp.ESSCertIDv2`
- Fixed a bug constructing a `cms.SignedData` object
- Ensure that specific RSA OIDs are always paired with `parameters` set to
`core.Null`

- Fixed DER encoding of `core.BitString` when a `_map` is specified (i.e. a
"named bit list") to omit trailing zero bits. This fixes compliance of
various `x509` structures with RFC 5280.
- Corrected a side effect in `keys.PrivateKeyInfo.wrap()` that would cause the
original `keys.ECPrivateKey` structure to become corrupt
- `core.IntegerOctetString` now correctly encodes the integer as an unsigned
value when converting to bytes. Previously decoding was unsigned, but
encoding was signed.
- Fix `util.int_from_bytes()` on Python 2 to return `0` from an empty byte
string