Implement fungible and fungibles for tokens

[zjb0807]

#417

currencies module is updating.

[xlc]

we shouldn't ignore keep_alive

[zjb0807]

open-runtime-module-library/tokens/src/lib.rs

Lines 519 to 538 in 7cd2c10

	fn transfer(
	currency_id: Self::CurrencyId,
	from: &T::AccountId,
	to: &T::AccountId,
	amount: Self::Balance,
	) -> DispatchResult {
	if amount.is_zero() || from == to {
	return Ok(());
	}
	Self::ensure_can_withdraw(currency_id, from, amount)?;
	let from_balance = Self::free_balance(currency_id, from);
	let to_balance = Self::free_balance(currency_id, to)
	.checked_add(&amount)
	.ok_or(ArithmeticError::Overflow)?;
	// Cannot underflow because ensure_can_withdraw check
	Self::set_free_balance(currency_id, from, from_balance - amount);
	Self::set_free_balance(currency_id, to, to_balance);
	Ok(())

Tokens transfer not handle the ExistenceRequirement

[xlc]

we need a version that handles it, and transfer can call it with keep_alive: false

[xlc]

this will break too many existing code.
I was thinking only update the transfer implementation of orml-tokens. Add a new helper method to take existence_requirement.

[xlc]

remove this as well