Inferno should support leading underscores and/or wildcard bindings

[ngua]

I tried implementing this in #174 but it made some inferno-core tests fail.

Currently, Inferno does not allow the following:

let _ = 1 in ()

let _x = 1 in ()

I don't really thing there's a compelling reason to not support these. Inferno ML now has side-effect only bindings (Print.print, Print.printWith), and it's annoying to have to use a dummy variable name. And Inferno LSP also warns on the unused variable.

Changing the parser to support this was not difficult, but it looks like more work would be needed in the inference stage.

Maybe as an intermediate step we could allow leading underscores only (and no wildcards) and have Inferno LSP not warn on them.