Add microsecond provider

[ahawker]

The microsecond provider uses more precise clock resolution to
create randomness values that should monotonically increase on
timestamp (millisecond) collision.

When creating a new ULID, we extract the number of microseconds from
our timestamp and use them as the first two bytes of the randomness value.

Caveats:

When using ulid.from_timestamp, we fallback to the default implementation
of 10 bytes of pure randomness. The reason for this, is that the timestamp
that is being passed in by the caller is in milliseconds. Since we don't have
a more accurate timestamp to "steal" bytes from, there's not much we can do. We
cannot make a new epoch timestamp as the microsecond remainder is only accurate
when working within the same millisecond.

Status: Ready

If merged, this adds a microsecond implementation as discussed in #306 .

TODO:

• Add tests for microsecond provider
• Add tests for microsecond api
• Add tests for new provider method

Usage:

>>> from ulid import microsecond as ulid
>>> ulid.new()
<ULID('01EH0VVVEC0BKJHF0370TNGQ4Z')>
>>> ulid.new()
<ULID('01EH0VVWPG0C6VDD0529CAHPNJ')>
>>> ulid.new()
<ULID('01EH0VVX8R0AN45DBYZZYMXVKT')>
>>> ulid.new()
<ULID('01EH0VVYA406BDKKRVCDJQZHYQ')>

[codecov]

Codecov Report

Merging #476 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #476   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        19    +2     
  Lines          498       547   +49     
  Branches        80        80           
=========================================
+ Hits           498       547   +49     

Impacted Files	Coverage Δ
ulid/api/default.py	100.00% <ø> (ø)
ulid/api/monotonic.py	100.00% <ø> (ø)
ulid/api/api.py	100.00% <100.00%> (ø)
ulid/api/microsecond.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/microsecond.py	100.00% <100.00%> (ø)

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