Description of changes

Fixes issues identified in our stream types related to stream length reporting vs. the actual number of bytes returned in streams.

• Buffered stream now throws and rejects additional data when attempting to write to a stream that is closed.

Also:

• Tests on types in SmithyStreams have been separated from ClientRuntimeTests into their own test target SmithyStreamsTests.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.