Add constrained_types package to the standard library

Implements RFC 79.

https://github.com/ponylang/rfcs/blob/main/text/0079-constrained-types.md

Closes #4492

packages/constrained_types/_test.pony

@@ -0,0 +1,8 @@
+use "pony_test"
+
+actor \nodoc\ Main is TestList
+  new create(env: Env) => PonyTest(env, this)
+  new make() => None
+
+  fun tag tests(test: PonyTest) =>
+    None

packages/constrained_types/constrained.pony

@@ -0,0 +1,37 @@
+type ValidationResult is (ValidationSuccess | ValidationFailure)
+
+primitive ValidationSuccess
+
+class val ValidationFailure
+  let _errors: Array[String val] = _errors.create()
+
+  new create(e: (String val | None) = None) =>
+    match e
+    | let s: String val => _errors.push(s)
+    end
+
+  fun ref apply(e: String val) =>
+    _errors.push(e)
+
+  fun errors(): this->Array[String val] =>
+    _errors
+
+interface val Validator[T]
+  new val create()
+  fun apply(i: T): ValidationResult
+
+class val Constrained[T: Any val, F: Validator[T]]
+  let _value: T val
+
+  new val _create(value: T val) =>
+    _value = value
+
+  fun val apply(): T val =>
+    _value
+
+primitive MakeConstrained[T: Any val, F: Validator[T] val]
+  fun apply(value: T): (Constrained[T, F] | ValidationFailure) =>
+    match F(value)
+    | ValidationSuccess => Constrained[T, F]._create(value)
+    | let e: ValidationFailure => e
+    end

packages/stdlib/_test.pony

@@ -24,6 +24,7 @@ use capsicum = "capsicum"
 use cli = "cli"
 use collections = "collections"
 use collections_persistent = "collections/persistent"
+use constrained_types = "constrained_types"
 use debug = "debug"
 use files = "files"
 use format = "format"
@@ -57,6 +58,7 @@ actor \nodoc\ Main is TestList
     cli.Main.make().tests(test)
     collections.Main.make().tests(test)
     collections_persistent.Main.make().tests(test)
+    constrained_types.Main.make().tests()
     files.Main.make().tests(test)
     format.Main.make().tests(test)
     ini.Main.make().tests(test)