Avoid loading entire file into memory in `fileInfo`

- [ ] In hackage-security: See TODO for `fileInfo`; currently we do two separate passes over the input bytestring, one to compute the length and one to compute the hash. Now that we have decided on the cryptolibrary to use, we should do this in a single pass. 
- [ ] In hackage-server: See TODO for `constructTUFFiles`
