wasm: dynamically dispatch data functions with "seen" ref pieces using call_indirect

[srenatus]

1. write an object corresponding to data paths into the module data
2. initialize an opa_object_t from that using _initialize, called
   as the module's Start function
3. write out CallDynamicStmts in IR when the ref is not all ground,
   but its vars have been seen
4. compile those CallDynamicStmts to call_indirect invocations in
   WASM, preceded by a lookup using the path in the object prepared
   in (2.)
5. if the lookup fails to come up with a result, the eval goes
   undefined

---

What it looks like:

With t.rego as

package t

p {
  data.foo[input.x].bar.p
}

and foo.rego as

package foo.a.bar

p = true

when building policy.wasm using opa build -t wasm -e t/p t.rego foo.rego,
the body of function g0.data.t.p will contain

i32.const 5
call $opa_array_with_cap
local.set $11
local.get $11
local.get $7
call $opa_array_append
local.get $11
local.get $8
call $opa_array_append
local.get $11
local.get $6
call $opa_array_append
local.get $11
local.get $9
call $opa_array_append
local.get $11
local.get $10
call $opa_array_append
local.get $0
local.get $1
local.get $11
call $opa_mapping_lookup
local.tee $12
i32.eqz
br_if $block
local.get $12
call_indirect $29 (type $1)
local.tee $13
i32.eqz
br_if $block

Where the array-related functions build an array of

["g0", "foo", input.x, "bar", "p"]

and pass that to opa_mapping_lookup to determine the element index to
pass to call_indirect. The lookup function returns 74 from the JSON
blob put into the data section,

(data $38 (i32.const 56485)
  "{\"g0\": {\"foo\": {\"a\": {\"bar\": {\"p\": 74}}}, \"t\": {\"p\": 75}}}")

iff input.x happens to be "a". Otherwise, it'll return 0, and the result
will end up being undefined.

Element 74 of the modules func table is, of course, $g0.data.foo.a.bar.p:

(elem $33 (i32.const 74)
  $g0.data.foo.a.bar.p $g0.data.t.p)

($33 is some id of that piece of function table, an artifact of the
wavm disassemble output.)

---

Fixes #2936.

[srenatus]

ℹ️ Pushed my latest WIP; the planner changes are not correct yet.

💣 On my machine at least, I had to resort to using v8 for testing (make wasm-rego-test); our golang SDK explodes with this branch, in the same way it does for the jsonpatch tests.

[srenatus]

✔️ Having _initialize called through the Start section mechanism seems to work fine. 😃

[srenatus]

Two more things to check:

• there's an extra block we might not need
  • it really doesn't matter. we have the same with CallStmt.
• there should be a test case involving with, just to be sure

Currently, the test cases don't really check that call_indirect was used; but they help (me) to poke through the WASM code, seeing if everything is in order.

[tsandall]

This looks great. The planner changes are quite clean! I've left a few comments.

[tsandall]

I could imagine it being useful to have the planner emit debug messages like compile/compile.go does for some of the optimizations it performs. Otherwise it becomes tricky to understand why the optimization hasn't kicked in.

[tsandall]

LGTM!