rename redefined variables

[ailrst]

Adds a transform pass that renames variables that have multiple definitions in the same scope.

Fixes #43

[katrinafyi]

i'm not sure if you've fixed these already.

it would be good if the indentation is fixed, and everything else is just commentary.

[katrinafyi]

This changeset slows dune runtest by 10 seconds in total, investigating. (also visible in CI where the change is more pronounced)

Edit: seems to be the globals initialisation within scoped_bindings.

      let globals =  Seq.map (fun f -> (f, {name=f; index=0})) (IdentSet.to_seq globals) in

pushing an empty IdentMap instead seems to work ok..? there shouldn't be any redeclarations of global variables.