core: experiment with reducing allocs for transfers

[MariusVanDerWijden]

Funny experiment, but I don't think we should merge it.
Will leave it as draft for a bit to discuss, then close it

  ┌───────────┬───────────┬─────────────────┬──────────────────┐
  │  metric   │ baseline  │ early fast path │      delta       │
  ├───────────┼───────────┼─────────────────┼──────────────────┤
  │ sec/op    │ 44.63 ms  │ 35.65 ms        │ −20.1% (p=0.002) │
  ├───────────┼───────────┼─────────────────┼──────────────────┤
  │ B/op      │ 36.78 MiB │ 18.98 MiB       │ −48.4% (p=0.002) │
  ├───────────┼───────────┼─────────────────┼──────────────────┤
  │ allocs/op │ 557.1k    │ 319.0k          │ −42.7% (p=0.002) │
  └───────────┴───────────┴─────────────────┴──────────────────┘