Linear types: fix varMult for let-bound variables

Linear types add a new field varMult to Id.

Currently, this field stores a multiplicity for lambda-bound variables and case binders, and an arbitrary value for let-bound variables (which is ignored, Note [Multiplicity of let binders]). Instead, varMult should be a Maybe which is Nothing if this value is not used. Going further, instead of Nothing, let-bound variables could store their aliased usage environment.