Runtime support for cel.@block

[TristonianJones]

Introduces runtimes support for cel.@block

CEL block is used by static optimizers to group subexpressions together
such that the following is true:

• Each subexpression value may be referenced by its ordinal through the
  variable: @index<ordinal>
• Subexpressions must be organized such that the expression at @index<N>
  may only depend on variables represented in @index<N-1>
• Index expressions may not occur within a comprehension loop since each
  comprehension represents a scope which declares its own variables.

Depends on google/cel-spec#402 before submission

[jcking]

I personally would just avoid the pointer here.

[TristonianJones]

The pointer ends up being pretty convenient since it means that I can update the slotVal state without having to set the slot index multiple times.

[TristonianJones]

Any other concerns? I've written it both ways and the visit logic is easier to follow with the pointers. Object pooling generally solves any overhead concerns. See #1056

[TristonianJones]

I wound up running tests against the pooled allocations versus stack-based allocs and overall the pointers with pooled memory were a 5% win, so I'm sticking with that for now and just merged the policy composition support. Happy to come back to this as a follow up if desired.