non-cast implementation of toBytes, add fromBytes

stew/byteutils.nim

@@ -127,8 +127,12 @@ func toHex*[N: static[int]](ba: array[N, byte]): string {.inline.} =
 func toBytes*(s: string): seq[byte] =
   ## Convert a string to the corresponding byte sequence - since strings in
   ## nim essentially are byte sequences without any particular encoding, this
-  ## is almost a noop
-  cast[seq[byte]](s)
+  ## simply copies the bytes without a null terminator
+  @(s.toOpenArrayByte(0, s.high))
+
+func fromBytes*(T: type string, v: openArray[byte]): string =
+  result = newString(v.len)
+  copyMem(addr result[0], unsafeAddr v[0], v.len)
 
 func `<`*(a, b: openArray[byte]): bool =
   ## Lexicographical compare of two byte arrays

tests/test_byteutils.nim

@@ -81,3 +81,9 @@ suite "Byte utils":
 
       c < d
       not (d < c)
+
+  test "strings":
+    check:
+      "a".toBytes() == @[byte(ord('a'))]
+      string.fromBytes([byte(ord('a'))]) == "a"
+      cast[ptr UncheckedArray[byte]](cstring(string.fromBytes([byte(ord('a'))])))[1] == byte(0)