Proposal: Separate Layer for Context Propagation (open-telemetry#66)

* Proposal to separate context propagation from observability

* cleanup description for Extract

* commas

Co-Authored-By: Christian Neumüller <christian+github@neumueller.me>

* Update text/0000-separate-context-propagation.md

Co-Authored-By: Christian Neumüller <christian+github@neumueller.me>

* RFC proposal: A layered approach to data formats

* whitespace

Co-Authored-By: Reiley Yang <reyang@microsoft.com>

* Capitalization

Co-Authored-By: Reiley Yang <reyang@microsoft.com>

* whitespace

Co-Authored-By: Reiley Yang <reyang@microsoft.com>

* CleanBaggage -> ClearBaggage

* move function descriptions to new line

* Add Optional subheader

* cleanup rough edits

* clean up advice on pre-existing context implementations

* Better context descriptions

* remove data format file

* remove git diff message

* improved code sytnax

* stop stuttering

* Update text/0000-separate-context-propagation.md

Co-Authored-By: Reiley Yang <reyang@microsoft.com>

* spacing

* Refine propagation

* Remove registry concept
* Add explicit chaining

* Add RFC ID number from PR

* remove RFC status line

* slight calrification for GetHTTPExtractor

* add global propagators

* Clean up motivation

* Clean up explanbation intro

* Clarify context types

* Fix ChainHTTPInjector and ChainHTTPExtractor

* typo

* Reference Trace-Context, not just traceparent

* Bagge context cleanup

* stronger language around context access

* Update text/0042-separate-context-propagation.md

Co-Authored-By: Christian Neumüller <christian+github@neumueller.me>

* clean up tradeoffs

* v2.0 of this OTEP

* Update OTEP number for new submission

* remove image file for unused diagram

* Update text/0066-separate-context-propagation.md

Co-Authored-By: alrex <alrex.boten@gmail.com>

* Link to Erlang prototype

* whitespace

* ToC

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Tristan Sloughter <t@crashfast.com>

* more context examples

* typo

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Yusuke Tsutsumi <tsutsumi.yusuke@gmail.com>

* Apply suggestions from code review

Co-Authored-By: Yuri Shkuro <yurishkuro@users.noreply.github.com>

* Renamed aspects to "cross-cutting concerns"

* injectors are a list instead of chained

* clean up API representation

* cleanup examples

* typo

* remove correlations from proposal

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Yuri Shkuro <yurishkuro@users.noreply.github.com>

* remove the term "chaining"

* remove the terms  upstream and downstream

* improved architecturural explanation

* whitespace

* update link to Go prototype

* Removed the Baggage API; replaced it with Correlations.

* Fix the intro paragraph for Correlations

* git merges ate my homework
* some sentences were out of order

* Clarify that correltions must be propagated

* Clarify risks

* removed extra header

* Clarify definition of aspect-oriented programming

* Fix RemoveCorrelation

* Spelling

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Sergey Kanzhelev <S.Kanzhelev@live.com>

* Clarifying details

* Update python prototype

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Tyler Yahn <MrAlias@users.noreply.github.com>

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Tyler Yahn <MrAlias@users.noreply.github.com>

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Tyler Yahn <MrAlias@users.noreply.github.com>

* Clarify that the APIs and example code are not meant as final.  Add C# prototype

* Inject returns headers, not context

* Update text/0066-separate-context-propagation.md

Co-Authored-By: Mauricio Vásquez <mauricio@kinvolk.io>

* spelling

* remove spurious go comment

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>
Co-authored-by: Reiley Yang <reyang@microsoft.com>
Co-authored-by: alrex <alrex.boten@gmail.com>
Co-authored-by: Tristan Sloughter <t@crashfast.com>
Co-authored-by: Yusuke Tsutsumi <tsutsumi.yusuke@gmail.com>
Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>
Co-authored-by: Sergey Kanzhelev <S.Kanzhelev@live.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Mauricio Vásquez <mauricio@kinvolk.io>