Deprecate array->string cast (#23380)

Deprecate casting arrays to string. This is a continuation of the
deprecation of the universal string cast from [this
PR](#22068). See
#19893 for more context.

This deprecation recommends replacing:
```chapel
var s = myArray: string;
```
with:
```chapel
var s = "%?".format(myArray);
```

- [x] paratest

[ reviewed by @riftEmber ] - Thanks!

modules/internal/ChapelArray.chpl

@@ -2210,6 +2210,7 @@ module ChapelArray {
 
   // How to cast arrays to strings
   @chpldoc.nodoc
+  @deprecated(notes="casting arrays to string is deprecated; please use 'try! \"%?\".format()' from IO.FormattedIO instead")
   operator :(x: [], type t:string) {
     import IO.FormattedIO.string;
     return try! "%?".format(x);

test/arrays/cast/cast-to-string.chpl

@@ -0,0 +1,16 @@
+var a = [i in 1..10] i;
+
+operator :(arr: [], type t:string) {
+  var s = "---";
+  var first = true;
+  for x in arr {
+    if first
+      then first = false;
+      else s += " ";
+    s += x:string;
+  };
+  s += "---";
+  return s;
+}
+
+writeln(a:string);

test/arrays/cast/cast-to-string.good

@@ -0,0 +1 @@
+---1 2 3 4 5 6 7 8 9 10---

test/arrays/cast/promoted-cast-to-string.chpl

@@ -0,0 +1,18 @@
+record R {
+  var x: int;
+
+  operator: (r: R, type t:string) {
+    return "-" + r.x:string + "-";
+  }
+}
+
+proc string.init=(r: R) {
+  init this;
+  this = r:string;
+}
+
+const d = {1..10},
+      a = [i in d] new R(i),
+      b: [d] string = a;
+
+writeln(b);

test/arrays/cast/promoted-cast-to-string.good

@@ -0,0 +1 @@
+-1- -2- -3- -4- -5- -6- -7- -8- -9- -10-

test/deprecated/IO/array-string-cast.chpl

@@ -0,0 +1,2 @@
+const x: [1..10] int;
+writeln(x:string);

test/deprecated/IO/array-string-cast.good

@@ -0,0 +1,2 @@
+array-string-cast.chpl:2: warning: casting arrays to string is deprecated; please use 'try! "%?".format()' from IO.FormattedIO instead
+0 0 0 0 0 0 0 0 0 0

tools/mason/MasonTest.chpl

@@ -591,11 +591,11 @@ proc runAndLog(executable, fileName, ref result, reqNumLocales: int = numLocales
   // (albeit wasteful) thing we can do here is just cast the lists to
   // array here.
   //
-  if testNames.size != 0 then testNamesStr = testNames.toArray(): string;
-  if failedTestNames.size != 0 then failedTestNamesStr = failedTestNames.toArray(): string;
-  if erroredTestNames.size != 0 then erroredTestNamesStr = erroredTestNames.toArray(): string;
-  if testsPassed.size != 0 then passedTestStr = testsPassed.toArray(): string;
-  if skippedTestNames.size != 0 then skippedTestNamesStr = skippedTestNames.toArray(): string;
+  if testNames.size != 0 then testNamesStr =                try! "%?".format(testNames.toArray());
+  if failedTestNames.size != 0 then failedTestNamesStr =    try! "%?".format(failedTestNames.toArray());
+  if erroredTestNames.size != 0 then erroredTestNamesStr =  try! "%?".format(erroredTestNames.toArray());
+  if testsPassed.size != 0 then passedTestStr =             try! "%?".format(testsPassed.toArray());
+  if skippedTestNames.size != 0 then skippedTestNamesStr =  try! "%?".format(skippedTestNames.toArray());
   var exec = spawn([executable, "-nl", reqNumLocales: string, "--testNames",
             testNamesStr,"--failedTestNames", failedTestNamesStr, "--errorTestNames",
             erroredTestNamesStr, "--ranTests", passedTestStr, "--skippedTestNames",