Revisit contract_apis support for stable toolchain

This would be much easier for contract devs to be able to just use stable toolchain. Nightly toolchain’s pinned versions are in the repo in a subdirectory, which is already hard to find, but when the toolchain version changes it’s unclear how developers can update their contracts. As part of this task we should revisit support for stable. And as a consequence stable likely needs proper support for std.