fix(sol-macro): snake_case'd function names

[DaniPopes]

Motivation

heck doesn't treat numbers as new words, and discards leading underscores, thus:

contract C {
  function f();
  function _f();
}

failed to compile because the generated is_... functions would have the same name for both variants.

Solution

Create a helper function for snakifying idents, taken from strum

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[prestwich]

just cliipy