Add proc toOpenArray[byte] for strings (#7820)

lib/system.nim

@@ -4164,7 +4164,8 @@ when not defined(js):
     magic: "Slice".}
   proc toOpenArray*(x: string; first, last: int): openarray[char] {.
     magic: "Slice".}
-
+  proc toOpenArrayByte*(x: string; first, last: int): openarray[byte] {.
+    magic: "Slice".}
 
 type
   ForLoopStmt* {.compilerProc.} = object ## special type that marks a macro

tests/system/tsystem_misc.nim

@@ -15,6 +15,16 @@ discard """
 1
 2
 3
+48
+49
+50
+51
+52
+53
+54
+55
+56
+57
 '''
 """
 
@@ -86,3 +96,9 @@ doAssertRaises(IndexError):
   foo(toOpenArray(arrNeg, -1, 0))
 doAssertRaises(IndexError):
   foo(toOpenArray(arrNeg, -1, -3))
+
+proc foo(a: openArray[byte]) =
+  for x in a: echo x
+
+let str = "0123456789"
+foo(toOpenArrayByte(str, 0, str.high))