I keep repeating the same Elixir code in my reporters, middleware, and unit
tests. This commit extracts all that code, cleans it up, and makes it
available for everyone else working with Opencensus in Elixir.

- **Alter `Opencensus.Trace.with_child_span/2` attribute handling**
- Add `Opencensus.Trace.put_span_attributes/1`
- Add `Opencensus.Trace.async/1`
- Add `Opencensus.Trace.async/3`
- Add `Opencensus.Trace.await/1`
- Add `Opencensus.Span` struct derived from the record
- Add `Opencensus.Span.from/1`
- Add `Opencensus.Span.load/1`
- Add `Opencensus.SpanContext` struct derived from the record
- Add `Opencensus.SpanContext.from/1`
- Add `Opencensus.SpanContext.hex_trace_id/1`
- Add `Opencensus.SpanContext.hex_span_id/1`
- Add `Opencensus.TestSupport.SpanCaptureReporter`
- Add `Opencensus.Attributes.process_attributes/1`
- Add `Opencensus.Attributes.process_attributes/2`
- Add `Opencensus.Attributes.default_attributes/1`

The new attribute handling:

- **Drops span attributes with unsupported values**

- **Flattens nested maps**

- Documents the value support limits imposed by the Opencensus wire protocol
  and `:ocp.put_span_attribute/2`

- Preserves the `:module`, `:file`, `:function`, `:line`, and `:default`
  magic attributes

Misc:

- `mix credo`
- `mix test.watch`