function isLetter(ch) {
  /*eslint no-bitwise:0*/
  var lc = ch | 0x20; // to lower case
  return (lc >= 0x61/* a */) && (lc <= 0x7a/* z */);
}
Ruler.getRules(chainName) -> Array

Return array of active functions (rules) for given chain name. It analyzes
rules configuration, compiles caches if not exists and returns result.

Default chain name is `''` (empty string). It can't be skipped. That's
done intentionally, to keep signature monomorphic for high speed.