Adding isNilOrEmpty and isNilOrWhitespace

As discussed in #4184, this patch adds `isNilOrEmpty` and
`isNilOrWhitespace` to `strutils`.

It also modifies the existing `isSpace` proc slightly to exit early
rather than looping through all characters in a string.

tests/stdlib/tstrutil.nim

@@ -95,5 +95,16 @@ assert(' '.repeat(0) == "")
 assert(" ".repeat(0) == "")
 assert(spaces(0) == "")
 
+assert(isNilOrEmpty(""))
+assert(isNilOrEmpty(nil))
+assert(not isNilOrEmpty("test"))
+assert(not isNilOrEmpty(" "))
+
+assert(isNilOrWhitespace(""))
+assert(isNilOrWhitespace(nil))
+assert(isNilOrWhitespace("       "))
+assert(isNilOrWhitespace("\t\l \v\r\f"))
+assert(not isNilOrWhitespace("ABc   \td"))
+
 main()
 #OUT ha/home/a1xyz/usr/bin