Mixed snake_case and camelCase in the codebase

[jgaskins]

The case conventions around method and argument names are inconsistent, which makes it difficult to remember whether I should use camelCase or snake_case for a given call.

The convention in Crystal is to use TitleCase (which some also call CamelCase, but that's confusing) for class/struct/module names and snake_case for everything else^*. Would you be open to a PR for this? I can attempt to keep existing calls backwards-compatible with deprecation warnings using the Deprecated annotation.

---

^*Constants and enum members are an exception to this, and they seem to be pretty split between TitleCase and SCREAMING_SNAKE_CASE, even in the stdlib. However, at least in the case of enums, we can use snake_case in most places anyway through automatic symbol->enum conversion (when passing an enum value as a method argument) and automatically generated enum predicate methods (like HTTP::Status#not_found?)