Uniswap v2: Use internal pairFor call rather than cross contract call

[HyunggyuJang]

wasm-showcase-dapps/uniswap-v2/logics/impls/router/router.rs

Lines 428 to 442 in 892ccc1

	/// Original Uniswap Library pairFor function calculate pair contract address without making cross contract calls.
	/// Please refer https://github.com/Uniswap/v2-periphery/blob/master/contracts/libraries/UniswapV2Library.sol#L18
	///
	/// In this contract, use cross contract call to get pair contract address.
	default fn _pair_for(
	&self,
	factory: AccountId,
	token_a: AccountId,
	token_b: AccountId,
	) -> Result<AccountId, RouterError> {
	let (token_0, token_1) = self._sort_tokens(token_a, token_b)?;
	let pair =
	FactoryRef::get_pair(&factory, token_0, token_1).ok_or(RouterError::PairNotFound)?;
	Ok(pair)
	}

We should use internal call to query pair address, which also will be a good showcase how to retrieve predetermined contract address like the one from create2 of EVM.