Make path steps type more formal

[frinyvonnick]

Prerequisites

• I have read the Contributing guidelines
• I have read the Code of conduct and I agree with it

Description

Make path steps type more formal

Issue : Fix #148

[codecov-io]

Codecov Report

Merging #156 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #156   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          77     78    +1     
  Lines         295    295           
=====================================
  Hits          295    295

Impacted Files	Coverage Δ
packages/immutadot/src/path/apply.js	100% <100%> (ø)	⬆️
packages/immutadot/src/core/get.js	100% <100%> (ø)	⬆️
packages/immutadot/src/util/protect.js	100% <100%> (ø)	⬆️
packages/immutadot/src/path/toPath.js	100% <100%> (ø)	⬆️
packages/immutadot/src/path/utils.js	100% <100%> (ø)
packages/immutadot/src/path/consts.js	100% <100%> (ø)

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[nlepage]

allowingArrays won't work anymore, and would become way more complex to maintain.
Do we keep it or discard it ?
If we discard it, toPath could become private...