Compare gas snapshots in CI with a small tolerance

The previous attempt to assert identical gas usage in CI has been
reverted because a snapshot generated in CI was different from a
snapshot generated locally.

The diff still exist as of today, but amounts are minuscule. The amounts
range from 1 to tens. And these diffs exist in fuzz tests only.

This is a known bug in Foundry: foundry-rs/foundry#5689

This commit adds back gas snapshot, and check for a minimum diff in CI.
This way, snapshot diff tool can be used while waiting for the Foundry
issue to be fixed.

Resolves issue Uniswap#216