This PR implements the currently in-review gRFC A113:
grpc/proposal#535. I've split the PR into
logically separate commits to help with the review process.

Summary of changes:
- Commit 1: simplify the implementation of `groupLocalitiesByPriority`
- Change the implementation to use newly added methods in the stdlib
`maps` and `slices` package to significantly simplify the implementation
(and get rid of an unnecessary test)
- Commit 2: Remove code that handles localities and endpoints of weight
0
- Remove unnecessary checks for locality and endpoint weights of `0` in
`cluster_resolver`. The xDS client already guarantees that these weights
will never be set to `0`.
- Commit 3: add the env var GRPC_EXPERIMENTAL_PF_WEIGHTED_SHUFFLING
- Commit 4: Weight computation changes in cluster_resolver LB policy
- This performs the weight normalization and fixed-point arithmetic
specified in A113
  - The change here is guarded by the above env var
- Ended up duplicating the tests that verify the weight computation
behavior. This will make it easier to delete the old tests when the env
var is removed.
- Commit 5: Fix a broken test in ring_hash due to the new weight
computation
- Commit 6: Weighted shuffling in pick_first
  - Contains the changes specified in A113 for the pick_first LB policy
  - Changes are guarded by the env var

RELEASE NOTES:
- pickfirst: Add support for weighted random shuffling of endpoints, as
described in gRFC A113