[main < E-path] Add path functions

[imilinovic]

Description

• path.slice - get a subset of a path,
  path.combine - combine two paths into one,
  path.elements - get path elements as a list of nodes and relationships PR368
• refactor the path module and change path.subgraph to bfs instead of dfs which improves the speed and lowers memory usage by a lot PR370

Pull request type

• Bugfix
• Algorithm/Module
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Related issues

Delete if this PR doesn't resolve any issues. Link the issue if it does.

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Reviewer checklist (the reviewer checks this part)

Module/Algorithm

• Core algorithm/module implementation
• Query module implementation
• Unit tests
• End-to-end tests
• Code documentation
• README short description
• Documentation on memgraph/docs
• Update GQLALchemy signatures in query builder using query module signature generator

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[imilinovic]

@vpavicic docs are here memgraph/documentation#274