fix string slice & splice (#5311)

code fixes courtesy of @memophen

tests/stdlib/tstring.nim

@@ -0,0 +1,55 @@
+discard """
+  file: "tstring.nim"
+  output: "OK"
+"""
+const characters = "abcdefghijklmnopqrstuvwxyz"
+const numbers = "1234567890"
+
+var s: string
+
+proc test_string_slice() =
+  # test "slice of length == len(characters)":
+  # replace characters completely by numbers
+  s = characters
+  s[0..^1] = numbers
+  doAssert s == numbers
+
+  # test "slice of length > len(numbers)":
+  # replace characters by slice of same length
+  s = characters
+  s[1..16] = numbers
+  doAssert s == "a1234567890rstuvwxyz"
+
+  # test "slice of length == len(numbers)":
+  # replace characters by slice of same length
+  s = characters
+  s[1..10] = numbers
+  doAssert s == "a1234567890lmnopqrstuvwxyz"
+
+  # test "slice of length < len(numbers)":
+  # replace slice of length. and insert remaining chars
+  s = characters
+  s[1..4] = numbers
+  doAssert s == "a1234567890fghijklmnopqrstuvwxyz"
+
+  # test "slice of length == 1":
+  # replace first character. and insert remaining 9 chars
+  s = characters
+  s[1..1] = numbers
+  doAssert s == "a1234567890cdefghijklmnopqrstuvwxyz"
+
+  # test "slice of length == 0":
+  # insert chars at slice start index
+  s = characters
+  s[2..1] = numbers
+  doAssert s == "ab1234567890cdefghijklmnopqrstuvwxyz"
+
+  # test "slice of negative length":
+  # same as slice of zero length
+  s = characters
+  s[2..0] = numbers
+  doAssert s == "ab1234567890cdefghijklmnopqrstuvwxyz"
+
+  echo("OK")
+
+test_string_slice()