…llowed with a 'trailer', e.g. zip()[x] (GH-24)

(cherry picked from commit 93b4b47)

Refactor into Python 3 and Python 2.7 submodules.

24: Warn for exception: move warning to ceval r=ltratt a=nanjekyejoannah

This replaces the old PR: softdevteam#12
Moved the warning to ceval.

I removed the three component warning because it was committed in an earlier PR here: softdevteam#14

Co-authored-by: Joannah Nanjekye <jnanjeky@unb.ca>

Implemented type propagation

This PR pretty much completely refactors the Rust build system. There are now two main ways Rust code is built:

# both

- The Rust triple is deduced based on the preprocessor run on Misc/platform_triplet.c
- A cpython-build-helper crate is used to pass the proper link arguments for each configuration
- The proper Rust toolchain is downloaded for iOS/Android/WASI for CI on those platforms
- cpython-sys is updated significantly to ensure bindgen properly generates the bindings for each platform

# shared

For shared builds, we build a crate into a cdylib and pass through link arguments from the makefile/configure and the linker executable so that the final link will match what is done for C programs.

# static

For static builds, a cpython-rust-staticlib crate is a built which depends on and re-exports the module initializers for each crate. This ensures there aren't duplicated Rust stdlib/core symbols.

Fixes python#23