Propagate arity between compilation units

[hhugo]

Fixes #550

• Store immutability property of blocks
• Try to recover some immutability info from debug events. (currently assuming modules are immutable.
• Use this info to compute an approximated shape for registered globals.
• Use shape info to compute function arity across compilation units
• Shapes are currently written as special comments (//# shapes:) inside the generated js and can be read back if explicitly requested (with the --load-shape FILE.cma.js)

TODOs:

• check the behavior with recursive modules
• fix with effects
• update global_flow ?
• better mechanism for shape propagation ?-
• store shape in a different file ? yes no, adding support in dune was too annoying.
• should the "shape" file be given jsoo on the command line or should it look for it on the file system ? both loading shapes need to be explicit.
• update dune rules with additional deps (Jsoo shapes ocaml/dune#10767)

[hhugo]

@vouillon, feel free to comment, review and even commit the branch.

[hhugo]

@vouillon, I think we could merge this before we have the support merged inside dune.
Would you want to give it another round of review ?

[hhugo]

It increases to the time to build the toplevel in whole program compilation by ~10%. But we don't benefit from shapes in such case.

[vouillon]

The following command spends a lot of time in Flow.the_shape_of (called from Specialize.function_arity).

js_of_ocaml `which ocamlc.byte` 

[hhugo]

@rickyvetter, the new/updated build rules should be much easier to integrate.
The PR now also works with wasm_of_ocaml.
More feedback/testing before merging would be nice. I'm planning to have this PR part of the next release.