Adding a UnifiedContext, composing DistributedContext and SpanContext.
This will enable propagators to extract and inject values from either system,
enabling more sophisticated schemes and standards to propagate data.

This also removes the need for generics and propagators that only
consume one or the other, requiring integrators to do extra work to
wire propagators appropriately.

Modifying the API of the propagators to consume the context as a mutable
argument. By passing in the context rather than returning, this enables the
chained use of propagators, allowing for situations such as supporting
multiple trace propagation standards simulatenously.

Migrating formatters outside of context. The design goal is to
allow context to be it's own object agnostic of the requirements
of specific telemetry. Instead, moving it under a broader propagators
module.

Removing UnifiedContext in lieu of directly using Context.
UnifiedContext adds an unneeded layer of abstraction that would
then require more operations to migrate into context.

Removing binaryformat and httptextformat modules specific to
SpanContext and DistributedContext. These are now handled by
a single formatter.

Removing the last few mentions of UnifiedContext

Fixing rst syntax of the context module.

Python3.4 and 3.5 do not support types on class attributes.

A Propagator class can help streamline integrations that require
extracting and injecting context.

Making the requests test pass again.

Fixing propagator impl in the SDK. The correct fields were not being
used.

In version of python before 3.7, a full import path involving a submodule
was failing tests.