[FEATURE REQUEST] Use more precise types to eliminate some casts

[vouillon]

Since we are using a uniform representation for values, we are making a lot of casts to recover a more precise type.

We should use a more precise type for the function environments to avoid some of these casts. The return type of functions could be refined as well.

Binaryen is probably refining the type of global variables already, so there is probably not much to gain there.

One could give a more precise type to runtime primitives. Wasm_of_ocaml would parse the runtime and insert appropriate casts in the generated code when needed. For instance:

(func (export "caml_array_append") (param $a1 (ref $block)) (param $a2 (ref $block)) (result (ref $block)))

It should be possible to refine the type of the parameters of some functions, when all their call sites are known.