Merlin_typer: fix typer of nested let bindings
Merlin_typer: fix typer of nested let bindings

chriskonstad/merlin,chambart/merlin,fxfactorial/merlin,dra27/merlin,chambart/merlin,ocaml/merlin,ocaml/merlin,the-lambda-church/merlin,haselwarter/merlin,ocaml/merlin,neojski/merlin,neojski/merlin,haselwarter/merlin,rgrinberg/merlin,fxfactorial/merlin,rgrinberg/merlin,fxfactorial/merlin,dra27/merlin,rgrinberg/merlin,dra27/merlin,chambart/merlin,haselwarter/merlin,the-lambda-church/merlin,fxfactorial/merlin,neojski/merlin,rgrinberg/merlin,the-lambda-church/merlin,dra27/merlin,neojski/merlin,chriskonstad/merlin,chambart/merlin,haselwarter/merlin,ocaml/merlin