[WIP] Add: Autoloading for toplevel constants

[Prajjwal]

Use Module#autoload to lazily load components.

Fixes #880 . Including parts of the gem independently is officially unsupported, and this should eliminate the need for doing that.

Will fix: #873 , hopefully by merging concurrent-ruby and concurrent-ruby-edge. Bundling extra features as edge is not required if dependencies are autoloaded.

Should also fix issues with circular dependencies, notably Concurrent::Options.

TODO:

• Autoload top level constants.
• Remove explicit requires in child components, components should use stuff autoloaded in the parent module.
• Test: Module methods and constants at boot line up exactly with previous versions of the library.
• Test & Benchmark in various ruby implementations.

[chrisseaton]

Do you still want to do this? Do you need any support?

Please rebase the branch.

[chrisseaton]

I'm closing this PR. Please feel free to come back to us in the future if you want to discuss again - we're very open to it.