Simplify sha1 functions

[ethantkoenig]

Some simplifications to prevent wheel re-invention:

• Use hex.EncodeString(..) instead of manually hex encoding
  • This is likely to be faster, since we don't have to allocate a byte array containg 012..ef each call.
• Use copy(..) instead of manually copying bytes
  • This is likely to be faster, since copy has architecture-specialized implementations
• Use bytes.Equal(..) instead of manually comparing bytes
  • Again, this is likely to be faster, since bytes.Equal has architecture-specialized implementations

For the record, converting an array to a slice (arr[:]) does not copy the array.

[lunny]

LGTM