TOML backend doesn't fully support TOML

Currently, the TOML support that was added supports simple Key/Value configurations but doesn't handle some of TOML's more complex structures.

We should:
- Define test cases for all TOML features
- Ensure that the TOML backend can pass those cases.

The spec for TOML can be found here: https://github.com/toml-lang/toml

The issue names no implementation files or tests. Read the TOML specification first, then locate the existing TOML backend and define test cases covering the missing structures and features; done means the backend passes those cases.

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