Promote mutables to refs when captured by closures

[kevmal]

Need to check on the erased case. But for the generated case

<@
    let mutable a = 0
    let f() = a <- a + 1
    f()
    a
@>

will return 0. In the short term there should at least be an error.

Otherwise perhaps the best place to fix this would be in QuotationSimplifier. Each mutable let could traverse down its body to check for references and then rewrite the Expr with a ref type if needed.