Remove file reading responsibility from loaders

[dylanholmes]

We will want to create a BaseFileManagerDriver to enable the FileManager Tool to interact with alternative (e.g. non-local) filesystems. We'd like to be able to reuse the existing loaders to load in artifacts from files, though some of the loaders used by the FileManager currently do not support loading files from memory, and thus would subclasses of BaseFileManagerDriver to write to a file before invoking the loader.

To make this easier, this change will remove the responsibility of reading from the filesystem from all loaders. Since this is the only reason the FileLoader loader exists, we will remove that.

Changes:

• Remove IO and Path type options as source. All loaders that are related to specific file formats will accept bytes as source (and str for text file formats).
• Remove FileLoader
• Edit FileManager to have no default loader and to return a BlobArtifact by default if there is no default loader. This is essentially the same behavior. (The default used to be FileLoader with no args, which would always return a BlobArtifact and if you changed the default loader to be FileLoader with encoding set, then it would be equivalent to passing a TextLoader for the default)
• Add to_key method on BaseLoader with a reasonable implementation and move load_collection implementation in to the BaseLoader. The idea here is to encapsulate the part of load_collection that varies. This allows there to be a single implementation that just submits load to the executor for each source.
• Remove duplicate loads in load_collection for sources that are listed more than once in the input (or at least that have the same key).
• Edit CHANGELOG

[collindutter]

Nice work!

[vasinov]

Looks great overall!