refactor(foreign): move implementations to dedicated, structured directory

[sivizius]

At the moment, the trait-definition and all implementations for foreign types are located in lib.rs with 1732 LOC. IMHO this is difficult to maintain, especially when adding additional implementations. By moving all implementations into the private submodule foreign, sorting them by core, alloc and std and submodules similar to those documented in e.g. https://doc.rust-lang.org/std/, I see i.a. the following advantages:

1. Easier to find for which types the trait is implemented, which implementations are missing,
2. If one wants to add a implementation, one can just place it in the module according to its type definition in core/alloc/std and does not have to guess, whether to place it at the beginning or end of lib.rs or somewhere random,
3. It will be easier to feature-gate some implementations, e.g. std for std-implementations, which should be a default-feature, which can be disabled for no_std-builds,
4. Implementations for additional (common) crates, e.g. chrono or time, can be placed in foreign behind feature-gates,
5. Changes to one file does not affect changes to other files, so merging multiple PRs might be less conflicting

However, this is indeed quite a large change and will require additional PRs e.g. for feature-gating, fixing the usage of std in alloc, adding missing implementations, ….

[fitzgen]

Thanks!