Attempt s2_simplify()

[paleolimbot]

Still sketching...basically trying to get the builder to build something. I was hoping to make this work for all dimensions at once, and I think I can based on the dimension attr of the S2Shape.

[codecov-commenter]

Codecov Report

Merging #70 into master will decrease coverage by 2.03%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   93.98%   91.95%   -2.04%     
==========================================
  Files          35       35              
  Lines        1780     1938     +158     
==========================================
+ Hits         1673     1782     +109     
- Misses        107      156      +49     

Impacted Files	Coverage Δ
R/s2-matrix.R	100.00% <ø> (ø)
R/s2-predicates.R	100.00% <ø> (ø)
src/s2-options.h	63.31% <65.13%> (-21.03%)	⬇️
R/s2-options.R	96.96% <100.00%> (+6.06%)	⬆️
R/s2-transformers.R	100.00% <100.00%> (ø)
src/s2-transformers.cpp	98.13% <100.00%> (+0.48%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 27afb99...e6fa9d6. Read the comment docs.

[paleolimbot]

@edzer Just a heads up that there's a breaking change here for code that used non-default s2_options(): options are now specified in "english" rather than numerically, and they are all specified explicitly (rather than falling back on the S2 defaults, which are difficult to track down). I made too many mistakes writing tests with the numeric values, so I assume actual users will do the same.