Implement ARRAY_TO_STRING built in function

ARRAY_TO_STRING(anyarray, text [, text]) - Concatenates array elements using supplied delimiter and optional null string and returns the resulting string

eg:
ARRAY_TO_STRING(ARRAY['1', '2, '3', NULL, '5'], ',', 'k') → '1,2,3,k,5'
ARRAY_TO_STRING(ARRAY[23, 34, 45], ',') → '23,34,45'