support cstring in case (#20130)

* implement case for cstring

for now just converts to string on C backend

* custom implementation for cstring

* remove leftover

* revert even more

* add nil + fix packages weird variant literal bug

* update docs

tests/casestmt/tcstring.nim

@@ -0,0 +1,52 @@
+discard """
+  targets: "c cpp js"
+"""
+
+type Result = enum none, a, b, c, d, e, f
+
+proc foo1(x: cstring): Result =
+  const y = cstring"hash"
+  const arr = [cstring"it", cstring"finally"]
+  result = none
+  case x
+  of "Andreas", "Rumpf": result = a
+  of cstring"aa", "bb": result = b
+  of "cc", y, "when": result = c
+  of "will", arr, "be", "generated": result = d
+  of nil: result = f
+
+var results = [
+  foo1("Rumpf"), foo1("Andreas"),
+  foo1("aa"), foo1(cstring"bb"),
+  foo1("cc"), foo1("hash"),
+  foo1("finally"), foo1("generated"),
+  foo1("no"), foo1("another no"),
+  foo1(nil)]
+doAssert results == [a, a, b, b, c, c, d, d, none, none, f], $results
+
+proc foo2(x: cstring): Result =
+  const y = cstring"hash"
+  const arr = [cstring"it", cstring"finally"]
+  doAssert not (compiles do:
+    result = case x
+    of "Andreas", "Rumpf": a
+    of cstring"aa", "bb": b
+    of "cc", y, "when": c
+    of "will", arr, "be", "generated": d)
+  case x
+  of "Andreas", "Rumpf": a
+  of cstring"aa", "bb": b
+  of "cc", y, "when": c
+  of "will", arr, "be", "generated": d
+  of nil: f
+  else: e
+
+results = [
+  foo2("Rumpf"), foo2("Andreas"),
+  foo2("aa"), foo2(cstring"bb"),
+  foo2("cc"), foo2("hash"),
+  foo2("finally"), foo2("generated"),
+  foo2("no"), foo2("another no"),
+  foo2(nil)]
+
+doAssert results == [a, a, b, b, c, c, d, d, e, e, f], $results