Nested object get and set not working in res files

[rizo]

Nested object set syntax is currently rejected with an assert false triggered at

melange/jscomp/frontend/ast_exp_apply.ml

Line 252 in f4ddebb

| _ -> assert false)

Input:

obj["foo"]["bar"] = 42

Error:

dune build @melange
File "src/ui/.ui.objs/melange/_unknown_", line 1, characters 0-0:
melc: internal error, uncaught exception:
      File "jscomp/frontend/ast_exp_apply.ml", line 252, characters 17-23: Assertion failed

Additionally, there's a related bug in nested gets.

x["foo"]["bar"]

Produces an incorrect compiled output:

x.bar

[jchavarri]

I got a repro in #533, seems the problem is somewhere in rescript conversion, instead of generating:

let () = ((entry ## person) ##  age) #= 99

it generates:

let () = ((entry ## person) #  age) #= 99