Some minor cleanup

- It's not necessary to take an address when calling a method
- v, _ := m[k] is equivalent to v := m[k] when m is a map
- k, _ := range ... is equivalent to k := range ...
- Use for...range in a few places
Use driver.RowsAffected type
Use IndexByte to find string terminator

No performance gain since bytes.Index special cases this anyway, but it means
stringTerm can go away.
Remove stmt.nparams

It was always equal to len(st.paramTyps) and therefore redundant.
Merge pull request #116 from edsrzf/cleanup

Some small cleanup