Memoize the Ace editor property callbacks.
Memoize the Ace editor property callbacks.

With the addition of the linting rules, we started running the editor
side effects whenever the callback passed to `useEditorProp` changed,
which caused inefficiency as well as unintended effects. This
manifested as the cursor jumping to the beginning of file after typing
any character.

Fixes #527

integer32llc/rust-playground,integer32llc/rust-playground,integer32llc/rust-playground,integer32llc/rust-playground,integer32llc/rust-playground