don't expose toml::Table in public API

[danieleades]

Problem

the toml::Table type is exposed in the public API (in the Config::get_preprocessor method), which means that changing the version of the toml dependency is a breaking change

Proposed Solution

hide the toml::Table value by returning a newtype, or by returning a impl Deserialize anonymous type

Notes

this change is itself a breaking change. Preprocessor developers who are accessing the Table values directly without deserialising into a custom struct will no longer be able to do so with the proposed change