builtin: add a rune iterator method to strings, allowing for for i, r in s.runes_iterator() { without first allocating an array for all the runes

[spytheman]

• builtin: add a s.runes_iterator() method, allowing for for i, r in s.runes_iterator() { with no allocations
• fix if_else_return.c.must_have

[huly-for-github]

Connected to Huly®: V_0.6-23186

[Delta456]

When to use .runes_iterator() and .runes()?

[spytheman]

When to use .runes_iterator() and .runes()?

If you need to use runes themselves in an array, use .runes() .

The iterator is useful, when you just need to iterate over all runes in a string, applying some computation on each rune in turn, without allocating an array in advance.