Parallelize build_wasm_samples.sh

[mversic]

All smart contracts are being built sequentially but they could be built in parallel. I believe this is a bottleneck in the CI and would also speed things up significantly locally

[mversic]

Also, I'm not sure that default_executor should be built along side all other smart contracts just because it sets a precedent where people might tend to build all smart contracts with build_wasm_samples.sh instead of just executor.wasm. I suggest separating it from the script