Add static method to create new Stream and allow instant chaining#2

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barteksc wants to merge 3 commits intobertptrs:masterfrom barteksc:master

Commits on Dec 22, 2018

Add static method to create new Stream and allow instant chaining
barteksc
committed
Add function returning first value from stream or default value
barteksc
committed

Commits on Dec 28, 2018

Add MappingCollector for mapping keys and values in collect phase
barteksc
committed