COMPILER: improve approximation

[hhugo]

extend the approximation of Param

example:

let s = "qwer"
let r = "poiu"
let f s = print_int (String.length s)
let _ = f s;f r

if f does't escape, we know that String.length s will always be 4

[hhugo]

@vouillon, any review ? can we merge ?

[vouillon]

By the way, there is probably an issue with inlining as well. In the example below, we know that parameter g is bound to function h, but we cannot inline g in f as x is not in scope.

let f g = g () in
let x = 1 in
let h () = x in
f h