Add separated namespace for document

[totakke]

Adds a new namespace, clucie.document, for document-related features.

I moved Document/Field/FieldType generators and Document -> map converter from clucie.core. Arities and docstrings of these functions were reconsidered, and now they can be easily used as public functions. In the near future, I'd like to add useful wrappers of sugar fields, such as StringField and SortedNumericDocValuesField.

[codecov-io]

Codecov Report

Merging #11 into master will increase coverage by 0.17%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage    88.7%   88.88%   +0.17%     
==========================================
  Files           4        5       +1     
  Lines         186      189       +3     
  Branches       14       14              
==========================================
+ Hits          165      168       +3     
  Misses          7        7              
  Partials       14       14

Impacted Files	Coverage Δ
src/clucie/core.clj	80.76% <66.66%> (-3.5%)	⬇️
src/clucie/document.clj	93.93% <93.93%> (ø)

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[federkasten]

Thank you. Looks good PR.

I have granted you a collaborator of clucie project. Could you help me to improve this library?

[totakke]

Thanks! I gladly accept your offer.