stable swap

[marco-sundsk]

introduce stable-swap pool into ref;

to know more about math logic, see ref-exchange/src/stable_swap/curve.md;

to know more about fee policy, see ref-exchange/src/stable_swap/fee_strategy.md;

to have a brief view about relevant interface, see ref-exchange/stable_swap.md;

for basic tests, see ref-exchange/src/stable_swap/mod.md::L682;

for simulation test, see ref-exchange/tests/test_stable_pool.rs;