Support R6 methods

[VisruthSK]

Fixes #14.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if ea9ae60 is merged into main:

• ❗🐌brms: 1.51s -> 1.57s [+3.61%, +4.65%]
• ❗🐌ggplot2: 1.22s -> 1.32s [+7.2%, +8.03%]
• ❗🐌loo: 541ms -> 549ms [+0.63%, +2.33%]
• ❗🐌projpred: 672ms -> 707ms [+4.6%, +5.86%]
• ❗🐌rstan: 403ms -> 419ms [+2.74%, +5.69%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8cac39f is merged into main:

• ❗🐌brms: 1.64s -> 1.72s [+2.53%, +6.79%]
• ❗🐌ggplot2: 1.34s -> 1.42s [+3.74%, +8.46%]
• ✔️loo: 584ms -> 583ms [-5.37%, +4.99%]
• ✔️projpred: 731ms -> 734ms [-5.18%, +6.15%]
• ❗🐌rstan: 435ms -> 450ms [+1.26%, +5.33%]
  Further explanation regarding interpretation and methodology can be found in the documentation.