Change indent creation for "Tries"

[nitkach]

Instead of iterating on Vec<usize> (i think this have O(n) complexity), use String.
Or create type around String, which will handle adding and deleting indent.

data_structures/src/bin/tries.rs

Lines 30 to 32 in 62d9363

	// make indent:
	// │ ...
	write!(f, "{}", indent.get_indent())?;