Add uniswap V2 WETH-USDC swap example #3267

Job	Run time
test no_std	18s
check serde	31s
clippy	1m 11s
check std	17s
check no-default-features	18s
docs	1m 11s
fmt	9s
test stable --no-default-features	1m 12s
test stable	1m 23s
test stable --all-features	1m 32s
test beta --no-default-features	1m 17s
test beta	1m 3s
test beta --all-features	1m 48s
test nightly --no-default-features	2m 23s
test nightly	2m 19s
test nightly --all-features	2m 43s
	19m 35s

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