Adds toSet to create sets from iterables (#16276)

tests/stdlib/tsetutils.nim

@@ -0,0 +1,14 @@
+discard """
+  targets: "c js"
+"""
+import std/setutils
+
+template main =
+  doAssert "abcbb".toSet == {'a', 'b', 'c'}
+  doAssert toSet([10u8, 12, 13]) == {10u8, 12, 13}
+  doAssert toSet(0u16..30) == {0u16..30}
+  type A = distinct char
+  doAssert [A('x')].toSet == {A('x')}
+  
+main()
+static: main()