Add monotonic randomness support

[ahawker]

Doing this required a refactoring of how timestamp/randomness values were generated. We've broken these into "provider" implementations with the "default" provider being the implementation that exists today, e.g. randomness values are random even on identical timestamp values.

A "monotonic" provider has been added which monotonically increments the first randomness value on timestamp collision until an overflow.

Additionally, the API has been broken out into a subpackage so we can stay agnostic to the provider and just plug it in. Work has been done to maintain the existing package interface for backwards compatibility.

This implementation should conform to the monotonic implementation found in https://github.com/ulid/spec even though there is discussion/concerns about it. See: https://github.com/ulid/spec/issues/40, https://github.com/ulid/spec/issues/11, https://github.com/ulid/javascript/issues/74

Status: Ready

If merged, this PR fixes #306.

TODO:

• [x] Add tests for api package
• [x] Add tests for providers package
• [x] Add tests for consts module
• [x] Add tests to assert default/monotonic api modules conform to public interface
• [x] Fix broken py35 tests
• [x] Fix linter errors

Usage:

>>> import ulid
>>> ulid.new()
<ULID('01EFZ5YBXG4G619GDRWESKW04C')>
>>> from ulid import monotonic as ulid
>>> ulid.new()
<ULID('01EFZ5YJ74PE8284B5M74CZBVW')>
>>> import time
>>> ts = time.time()
>>> ulid.from_timestamp(ts)
<ULID('01EFZ62V7VTEQR4Q788PSBBQP8')>
>>> ulid.from_timestamp(ts)
<ULID('01EFZ62V7VTEQR4Q788PSBBQP9')>
>>> ulid.from_timestamp(ts)
<ULID('01EFZ62V7VTEQR4Q788PSBBQPA')>
>>> ulid.from_timestamp(ts)
<ULID('01EFZ62V7VTEQR4Q788PSBBQPB')>
>>> ulid.from_timestamp(time.time())
<ULID('01EFZ648ZWCY8FEVYTN1YKBZXG')>

[codecov[bot]]

Codecov Report

Merging #473 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #473   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6        13    +7     
  Lines          368       467   +99     
  Branches        77        79    +2     
=========================================
+ Hits           368       467   +99     

Impacted Files	Coverage Δ
ulid/api/__init__.py	100.00% <100.00%> (ø)
ulid/api/api.py	100.00% <100.00%> (ø)
ulid/api/default.py	100.00% <100.00%> (ø)
ulid/api/monotonic.py	100.00% <100.00%> (ø)
ulid/consts.py	100.00% <100.00%> (ø)
ulid/hints.py	100.00% <100.00%> (ø)
ulid/providers/__init__.py	100.00% <100.00%> (ø)
ulid/providers/base.py	100.00% <100.00%> (ø)
ulid/providers/default.py	100.00% <100.00%> (ø)
ulid/providers/monotonic.py	100.00% <100.00%> (ø)
... and 9 more

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