Reorder the `mask` parameter to the end with a default of `_FULL_WARP_MASK`
for shfl_xor/down/up/sync, any_sync, all_sync, ballot_sync, and
match_any/all_sync. Full-warp is the overwhelmingly common case, so making
mask an optional trailing kwarg removes boilerplate at call sites and lets
old `(value, delta)`-style callers keep working. Update the test suite and
docs/instructions.md to match the new signatures.
Lei Wang
b3d5981b00