Add missing stubs for fully implementing the Span and Scope classes
from OpenTracing.

- Explicitly use opentracing.* to make OpenTracing imports easily
distinguishable.
- Leave OpenTelemetry class names as-is.

Since start_span() is a context manager and since we want the
OpenTelemetry Span to stop when the SpanWrapper stops, we need to
turn start_active_span() into a context manager and yield the
ScopeWrapper rather than return it.

Due to the differences between decimal and binary float
representations, some time values yield "surprising" results when
performing float arithmetic operations to convert seconds to
nanoseconds and vice versa.

"ot" isn't clear enough and could lead to confusion.

If the user passes a custom ScopeManager, we want the TracerWrapper
to call `active_span` on the custom ScopeManager.

Log a warning instead in order not to break application code.

Let the OTel implementation create event timestamps instead of
generating a timestamp in the shim, i.e. outside of OpenTelemetry.

- Remove an unnecessary context manager layers.
- Allow creating `ContextWrapper` objects either from a `SpanWrapper`
  or from a `Span` context manager.

Until we figure out how to handle unsupported frameworks (e.g.
gevent), we remove the `scope_manager` argument of `create_tracer`
to avoid breaking the OpenTelemetry context propagation.

These are obvious and therefore unnecessary.

The API docs are clear about the `references` argument being a list.
We don't try to compensate for wrong argument types elsewhere, so no
reason to do that here.

Not all classes in the shim are actually wrappers, which makes the
XWrapper naming convention confusing. XShim is more neutral.

In the OpenTracing API, when an exception occurs while a span is
active, information about the exception is added to the span as logs
and a tag is added as well. We re-add this functionality to the shim
since we've overridden the `__exit__()` method in which it lives.

Validating the parent type complicates the logic of `start_span()`.
Since the OpenTracing API clearly specifies the valid types for a
parent span, it's the user's responsibility to specify a valid
parent.

Verify that calling `close()` on a `ScopeShim` deactivates the span
in the OpenTelemetry tracer.

We shouldn't return OpenTelemetry objects to the OpenTracing API.

Make it clearer that the provided tracer object should be an
OpenTelemetry tracer, not an OpenTracing tracer.