This PR adds the ability to specify a name for the initialization
function of a Lean plugin on load.
* The `Lean.loadPlugin` API has gained a `initFn?` argument that
defaults to `none`. When `none`, the initialization function name will
be inferred from the shared library's name (as before).
* The CLI `--plugin` option can now have a initialization function
specified via `--plugin=path:initFn`.
* The `--setup` JSON configuration now also accepts`{"path": ...,
"initFn": ...}` for plugins.
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