Prepare loaders for TOML

[ziima]

What's the problem this feature will solve?

Prepare Loader and MemoryLoader for support of TOML files #999. Extracted from MR #3309.

Describe the solution you'd like

• Update Loader.__repr__ to match the implementation of IniLoader.__repr__.
• Standardize MemoryLoader.__init__ to accept configuration data as a single argument (not **kwargs), add section and overrides arguments to match Loader API.
• Update MemoryLoader.to_env_list to process sequences as value.

Alternative Solutions

None