We can move all promise ID knowledge to deno_core, allowing us to better
experiment with promise implementation in deno_core.
`{un,}refOpPromise(promise)` is equivalent to
`{un,}refOp(promise[promiseIdSymbol])`
This commit removes "Deno.core" namespace. It is strictly private API
that has no stability guarantees, we were supposed to remove it long time ago.
Co-authored-by: Yoshiya Hinosawa <stibium121@gmail.com>
