Problem:
When using `vim.list.unique` or `vim.list.bisect`, if the `key` function is
complex, it can degrade performance, because it is invoked on every comparison
Solution:
The `key` interface convention is designed specifically to address this issue;
performance can be improved by memoizing its results.
Also added the shorthand use of the field name string as the key.
97de5f145a
