Switch TOML implementation to toml_edit

[sunshowers]

toml_edit looks like the future of toml parsing and serialization in Rust: its design avoids ValueAfterTable issues, provides greater control over formatting if necessary, etc. I think it makes sense to switch to it at some point. Filing this task to keep track of blocking issues.

• easy::to_string_pretty doesn't produce pretty results toml-rs/toml#192