Separate number by commas, format phone

Separate number by comma with using `gsub`:

def separate_by_comma(number)
  number.to_s.reverse.gsub(/(\d{3})/, '\1,').reverse
end

without:

def spearate_by_comma(number)
  number.to_s.chars.reverse.each_slice(3).map(&:join).join(",").reverse
end

Rails: STI (Single Table Inheritance)

Rust: macros notation

Rust: some notation about Trait methods