Use ppx_deriving for transparent toml serialisation

[mackwic]

See: https://github.com/whitequark/ppx_deriving

We should not provide it in the main package (hide it behind a build flag) as some users don't want libraries to add unexpected syntax extension.

[sagotch]

:+1:

[redianthus]

Is it still useful as we have cconv support now ? @c-cube

[c-cube]

I have absolutely no idea.

[redianthus]

I'm not sure to get the idea. You could just store the string representation of your TOML datas or use Marshal ? I'm closing...