crypto/x25519: Add type-safe X25519 private/public key types

[peternose]

Waiting for oasisprotocol/curve25519-voi#120 to be merged.

[codecov]

Codecov Report

Merging #5121 (19a7334) into master (541dc2f) will decrease coverage by 0.04%.
The diff coverage is 54.09%.

❗ Current head 19a7334 differs from pull request most recent head 0f49a44. Consider uploading reports for the commit 0f49a44 to get more accurate results

@@            Coverage Diff             @@
##           master    #5121      +/-   ##
==========================================
- Coverage   66.84%   66.80%   -0.05%     
==========================================
  Files         497      496       -1     
  Lines       53273    53263      -10     
==========================================
- Hits        35611    35583      -28     
- Misses      13321    13334      +13     
- Partials     4341     4346       +5     

Impacted Files	Coverage Δ
go/keymanager/api/api.go	80.64% <ø> (ø)
go/runtime/host/protocol/types.go	54.54% <ø> (ø)
...consensus/tendermint/apps/keymanager/keymanager.go	62.05% <18.18%> (-1.59%)	⬇️
go/oasis-node/cmd/keymanager/keymanager.go	47.38% <36.36%> (-0.33%)	⬇️
go/runtime/registry/host.go	70.11% <88.00%> (-2.25%)	⬇️
go/common/crypto/mrae/deoxysii/asymmetric.go	77.77% <100.00%> (ø)
go/consensus/tendermint/apps/staking/auth.go	62.96% <0.00%> (-7.41%)	⬇️
go/runtime/host/sandbox/sandbox.go	70.00% <0.00%> (-6.56%)	⬇️
go/common/cbor/codec.go	78.37% <0.00%> (-5.41%)	⬇️
go/runtime/host/protocol/connection.go	68.79% <0.00%> (-5.27%)	⬇️
... and 27 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.