Don't parse and reserialize toml in configure.py

[jyn514]

Currently, configure.py has a lot of code dedicated to parsing and serializing toml: https://github.com/rust-lang/rust/blob/6959dd0ee3e8c0c11d3ea54b258dfaf24f148a49/src/bootstrap/configure.py#L382-L509
90% of this is unnecessary: when we get --set rust.foo as an argument, we can write rust.foo = true directly to config.toml, without needing to parse the toml itself. That should make configure.py a lot simpler, and as a side effect it means the hack to append a profile will be valid toml again: #112281 (comment)

@rustbot label E-easy E-mentor

[eval-exec]

we can write rust.foo = true directly to config.toml, without needing to parse the toml itself

Which toml file you mean in "without needing to parse the toml itself"? Is config.toml or config.example.toml?

[jyn514]

The toml in the --set string.

[eval-exec]

we can write rust.foo = true directly to config.toml

I think your idea is suitable for "appending a new toml config to config.toml", but it's hard to replace an exist config in config.toml

[eval-exec]

Oh, when ./config --set [balabala] is executated, we are creating a new config.toml file, not edit an existing file.

[abylaw]

I'd love to work on this as a first issue @rustbot claim

@jyn514, if you think there's anything worth mentioned before I get started, please feel free to me know!