Link field to store absolute and relative URI, including URL and URN.

[amcgregor]

Must be able to store:

• Complete URLs, including all URL components.
  http://user:pass@host:port/path?query#fragment

• Valid non-HTTP URI, including URN.
  mailto:user@example.com
urn:ISBN0-486-27557-4

• Incomplete or partial URLs in various formats.
  //example.com/protocol/relative
/host/relative
local/relative
#fragment-only

Additional changes:

• Field refactoring into independent modules.
• Set field. #37 — Set field definition.
• Field and index adaption. #38 — Field and index adaption.

[codecov]

Codecov Report

Merging #39 into develop will decrease coverage by 0.05%.
The diff coverage is 99.85%.

@@             Coverage Diff             @@
##           develop      #39      +/-   ##
===========================================
- Coverage      100%   99.94%   -0.06%     
===========================================
  Files           30       49      +19     
  Lines         1537     1773     +236     
===========================================
+ Hits          1537     1772     +235     
- Misses           0        1       +1

Impacted Files	Coverage Δ
marrow/mongo/core/field/base.py	100% <100%> (ø)	⬆️
marrow/mongo/core/field/ttl.py	100% <100%> (ø)
marrow/mongo/core/field/reference.py	100% <100%> (ø)
marrow/mongo/core/field/link.py	100% <100%> (ø)
marrow/mongo/core/field/boolean.py	100% <100%> (ø)
marrow/mongo/core/field/double.py	100% <100%> (ø)
marrow/mongo/core/field/date.py	100% <100%> (ø)
marrow/mongo/core/field/period.py	100% <100%> (ø)
marrow/mongo/core/field/decimal_.py	100% <100%> (ø)
marrow/mongo/core/field/oid.py	100% <100%> (ø)
... and 36 more

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