Memoize array of drop getter method names

[ashmaroli]

• This is an ⚡ optimization change.
• I've added tests.

Summary

Instead of stringifying array of drop-instance-methods for every drop instance, compute them once per drop class and reuse the resulting array of strings.

[ashmaroli]

Third-party Repo Memory Profile Summary

--- master https://github.com/jekyll/jekyll/runs/1241364408
+++ PR     https://github.com/jekyll/jekyll/runs/1263735861

- Total allocated: 224.63 MB (1271956 objects)
- Total retained:  40.74 MB (130038 objects)
+ Total allocated: 204.96 MB (1193919 objects)
+ Total retained:  40.77 MB (130586 objects)

[DirtyF]

LGTM, tests pass, we can see a slight faster build on the third-party sample repository. 👏

[ashmaroli]

Thank you Frank :)
@jekyllbot: merge +minor