Add simplified implementation in subproject/simple

[tobybell]

Regarding #64; here's my first stab at simplifying your reference implementation. It's still not exactly short and sweet, but it's at least a bit less noisy with macros and templates.

This implementation supports both to_decimal and to_chars, and the binary_to_decimal_rounding, decimal_to_binary_rounding, and cache policies. It's all in one file, simple_dragonbox.h.

It also has its own test executable, simple_dragonbox_test.cpp, which implements the same tests as uniform_random_test and test_all_shorter_interval_cases from the primary implementation.

I arrived at this implementation mostly by refactoring your existing code, rather than writing a new implementation from scratch, so there may still be simplifications I didn't spot. Especially your main algorithm, which I essentially didn't touch at all.

I'm submitting this as what I've got so far, since I need to pause working on it for now. From your comment on issue #64, I think there are a few further changes you might prefer to make:

• Remove support for policies altogether, and just use the default ones
• Reuse the same unit test code between the primary implementation and the simple implementation

I might return to work on those in the future. Let me know if there's anything else you see here that I missed as a simplification you had in mind. I wasn't sure how much you wanted to remove special cases for e.g. division by powers of 10, etc.

[jk-jeon]

Overall, looks pretty good. Thanks a lot!
There are just a few things that does not align with my preferences.

[tobybell]

Thanks for the prompt and detailed review. All your requested changes sound fine to me. I will implement them tomorrow (Monday) night.

[tobybell]

I think I addressed each of your comments with one commit, in order.

[jk-jeon]

Merged, thanks!

[jk-jeon]

Just a few notes about commits I made after merging the PR (not the ones listed above):

• I decided to wrap everything inside jkj:: namespace.
• I added license boilerplates into the source files.
• I removed your test file and rather merged it into the existing tests.
• Explicit template specialization is forbidden in class. You can only do partial specializations. A typical workaround is to add a dummy template parameter, so I did that for cache_holder.
• Added a mention of this in the main README.

Thanks again!