Implement `getPair()` and `allPairs()` functions

[gluneau]

A request to Implement getPair(address tokenA, address tokenB) and allPairs(uint) functions came from the Orion Team

[zjb0807]

I think we have the getPair

predeploy-contracts/contracts/dex/DEX.sol

Lines 35 to 39 in ea5682f

	/**
	* @dev Get Liquidity token address.
	* Returns (liquidity_token_address). Return address(0x0) if the liquidity token address is not mapped.
	*/
	function getLiquidityTokenAddress(address tokenA, address tokenB)

[gluneau]

https://docs.uniswap.org/protocol/V2/reference/smart-contracts/factory#getpair

[gluneau]

https://docs.uniswap.org/protocol/V2/reference/smart-contracts/factory#allpairs

[ferrell-code]

Also, I think allPairs() is not possible to do identically to uniswap because we don't have an index (0-n) mapped to pairs enabled

[xlc]

We need a precompile to iterate enabled pairs and return them.
However, not 100% sure why do we need such method?