Allow attach meta function to enum

[codehz]

Like __tostring so we can print enum easily

[edubart]

Support for this was added in commit 2c86ae0
Here is an example on how to use:

require 'string'

local MyEnum: type = @enum{A = 0, B}

function MyEnum.__tostring(value: MyEnum): string
  ## for i,field in ipairs(value.type.fields) do
  if value == #[field.value]# then
    return string.copy(#[value.type.nickname..'.'..field.name]#)
  end
  ## end
  return string.copy('Unknown')
end

print(MyEnum.A) -- outputs: MyEnum.A

Note that I've used string.copy because tostring must return a new allocated string, as mentioned in #101