[WIP] Move to core+alloc

[daviddrysdale]

Fixes #7

[daviddrysdale]

This currently still pulls in std because:

• tink-signature depends on ed25519-dalek ^1.0.1
  • ed25519-dalek 1.0.1 depends on rand_core ^0.5, we (separately) turn on the getrandom feature
    • rand_core 0.5.1 depends on getrandom ^0.1
      • getrandom 0.1.16 pulls in std on most common platforms, even when the std feature is absent