Vendor Base.Ryu

[nhz2]

From what I can tell, Base.Ryu is internal, so it may be removed in a future Julia version.

This PR vendors that code and other internal functions from Base that Ryu uses.

[codecov]

Codecov Report

❌ Patch coverage is 90.12075% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.09%. Comparing base (3f7d15f) to head (69109d4).

Files with missing lines	Patch %	Lines
src/ryu/shortest.jl	86.54%	46 Missing ⚠️
src/ryu/fixed.jl	87.91%	18 Missing ⚠️
src/ryu/exp.jl	91.00%	17 Missing ⚠️
src/ryu/Ryu.jl	91.66%	6 Missing ⚠️
src/write.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
+ Coverage   89.89%   90.09%   +0.20%     
==========================================
  Files           7       12       +5     
  Lines        1316     2221     +905     
==========================================
+ Hits         1183     2001     +818     
- Misses        133      220      +87     

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[quinnj]

I don't think this is a good idea to vendor:

• The Base Ryu code is still evolving/maturing; there have been at least 6 PRs this year massaging the code
• Vendoring means now we have a much higher code maintenance burden to try and keep synced w/ Base
• I think we can do something much simpler by doing a compat check on the presence of Base.Ryu and do standard printf calls otherwise.

I'm more worried about the long-term maintenance of this vendored code than Base breaking or slight performance hit of doing printf via compat.