Improve range support

// Range expression                              1st value          2nd value
// [x] array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E
// [ ] string          s  string type            index    i  int    see below  rune
// [ ] map             m  map[K]V                key      k  K      m[k]       V
// [ ] channel         c  chan E, <-chan E       element  e  E

• range over maps
• range over strings
• range over channels