Typesafe DSL. Reworked the whole API and deprecate old API (#16)

* first reworking of the whole API

* hide demux

* restoring graph API even if compiler fails

* removed demux api

* Designed API

* reflected providers

* moved pipes package to internal package

* moved everything to pipe package again

* renamed mid to middle and end to final

* seeming-to-work basic API

* ported back the removed libraries for safer migration

* Added deprecation notes in old libraries

* ported tests

* some lints

* fixed bug with nillable start and end providers

* removed documents

* fixed Final/Receiver confusion

* document builder.go

* keep going with documentation

* documented providers.go

* first version of the documentation

* starting minigrep example tutorial

* Changed readme

* Fixed bug during override

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Co-authored-by: mariomac <[email protected]>

updated formatter and reformatted

Version 0.8.0

Providers receive a context, and might return error (#9)

Multiple functions in start nodes (#8)

* AsStart and AsStartCtx now accept multiple functions that work as a single node

* MultiStartCtx also in Graph API

tutorial 03 and v0.4.2

added missing tutorial

documented public API