Add trim/0, ltrim/0 and rtrim/0 that trims leading and trailing whitespace (#3056)

tests/man.test

@@ -602,6 +602,12 @@ combinations(2)
 ["fo", "foo", "barfoo", "foobar", "foob"]
 ["fo","","bar","foobar","foob"]
 
+trim, ltrim, rtrim
+" abc "
+"abc"
+"abc "
+" abc"
+
 explode
 "foobar"
 [102,111,111,98,97,114]