Split out MultiLingual from Toolchain
Remove most C++ support

Needs to be reimplemented in the right layer
Renamed Autodetect to Autodetect::C
Make AutoDetect::C more easily overridable
Add basic C++ support
Refactor Command not to be parametric
Unconfuse PrereqScanner by being explicit
Use strictures
Remove Compiler::GCC

It didn't add anything other than subtly flawed defaults
Tidied code
Add tests for bottom layers and Compiler::Unixy
Encapsulate cc and ld