Refactor build index funcs

[katcipis]

Just a small refactoring inspired on the previous nested objects indexing stuff :-)

[codecov-io]

Current coverage is 54.15%

Merging #30 into master will change coverage by +0.46% by c7cd35a

Coverage Diff

@@            master     #30   diff @@
======================================
  Files           20      20       
  Stmts         1842    1819    -23
  Branches         0       0       
  Methods          0       0       
======================================
- Hit            989     985     -4
  Partial          0       0       
+ Missed         853     834    -19

---

Uncovered Suggestions

1. +1.38% via ...arch/index/filter.go#65...89
2. +1.32% via ...b/neosearch/index.go#215...238
3. +1.32% via ...arch/index/filter.go#10...33
4. See 7 more...

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