contract-sdk: Once wasm abi becomes stable consider switching back to it

[ptrus]

https://github.com/oasisprotocol/oasis-sdk/pull/548 switches from the exprimental wasm to using "C" ABI in the contract-sdk.

In future (as wasm abi becomes more stable) switching back to wasm ABI should be reconsidered. This however will be a breaking change for existing contracts, unless both ABIs are supported by the sdk.

Couple of issues with the wasm ABI:

• problems with u128 https://github.com/rust-lang/rust/issues/88207
• problems with compiling multivalue functions https://github.com/ptrus/wasm-bug (todo: should probably open an upstream issue for this)