Api
A loaded runtime: the shared library plus its abi-version api table.
An Api is a HANDLE to one loaded runtime: Api::clone_handle yields
a second handle to the same table, and the library stays mapped until
the last handle drops. A Tensor borrows the handle that produced it
(Tensor<'a> is a borrow type), so a handle a callback owns is what
lets that callback build tensors after the registering borrow has
ended (see Api::clone_handle).