Positions of locally bound variables in binder

[denismerigoux]

Right now, EVar nodes in the AST are accompanied with the source code position of the use of the variable in the source code. However, there is no way to link the Evar back to the position of its binder. This is annoying for the LSP as we cannot implement the "Go to definition" feature for locally bound variables (let ... in, aggregation operators, etc.). This is not high-priority for the moment.

Solutions to this problem include adding the binder position directly in the EVar node, or (more daunting) revisit the polymorphic bindlib encapsulation of variables to add the binder position inside it.