Ticket 386: Merge `write-bytevector` and `write-bytevector-partial`

Under this proposal, the signature would be (write-bytevector bytevector start end port), with the last three arguments optional. The only disadvantage here is that you would no longer be able to specify (write-bytevector bytevector port) directly; you'd need to say (write-bytevector bytevector 0 (bytevector-length bytevector) port).