encoding.base58: fix notice for slice creation

[sibkod]

>> compilation failed:
/opt/v/vlib/encoding/base58/base58.v:90:12: notice: an implicit clone of the slice was done here
   88 | 
   89 |     // now encode the values with actual alphabet in-place
   90 |     val := out[i - zcount..]
      |               ~~~~~~~~~~~~~~
   91 |     sz = val.len
   92 |     for i = 0; i < sz; i++ {
Details: /opt/v/vlib/encoding/base58/base58.v:90:12: details: To silence this notice, use either an explicit `a[..].clone()`,
or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.
   88 | 
   89 |     // now encode the values with actual alphabet in-place
   90 |     val := out[i - zcount..]
      |               ~~~~~~~~~~~~~~
   91 |     sz = val.len
   92 |     for i = 0; i < sz; i++ {