oldmap: use &K instead of K in find and get

This reverts commit a4250a96fdf61142a9c8dbb6d37ae8435c99e396.

This is not the cause of the nonexhaustive-match failure.
rt: Add rust_try_get_current_task
Make foreign calls work outside of tasks. #4451
Rewrite the exchange allocator to work without an active scheduler. #4457