Avoid `a` and `b` variable names.
Adjust `camelCase`, `kebabCase` and `snakeCase` doc examples. [ci skip]
Doc tweaks. [ci skip]
Update Underscore license year. [ci skip]
More doc cleanup. [ci skip]
Simplify `chain`.
Ensure sparse arrays are treated as dense by `keys` and `keysIn` and ensure support for string objects in `keys` and `keysIn`.