The interface between the host and guest is implemented as an
set_hdk function globally sets a
RefCell to track the current HDK implementation.
mock feature is set then this will default to an HDK that always errors, else a WASM host is assumed to exist.
mockall crate (in prelude with
mock feature) can be used to generate compatible mocks for unit testing.
See mocking examples in the test WASMs crate, such as
mockfeature, the host will be assumed.