Rename ToStringCoder into ToBytesCoder

The name of ToStringCoder class [1] is confusing, since the output of encode() on Python3 will be bytes. On Python 2 the output is also bytes, since bytes and string are synonyms on Py2.

ToBytesCoder would be a better name for this class.

Note that this class is not listed in coders that constitute Public APIs [2], so we can treat this as internal change. As a courtesy to users who happened to reference a non-public coder in their pipelines we can keep the old class name as an alias, e.g. ToStringCoder = ToBytesCoder to avoid friction, but clean up Beam codeabase to use the new name.

[1] https://github.com/apache/beam/blob/ef4b2ef7e5fa2fb87e1491df82d2797947f51be9/sdks/python/apache_beam/coders/coders.py#L344

[2] https://github.com/apache/beam/blob/ef4b2ef7e5fa2fb87e1491df82d2797947f51be9/sdks/python/apache_beam/coders/coders.py#L20

cc: yoshiki.obata chamikara