Simplify assertion machinery in the standard library.

This change includes a number of simplifications that allow us to
eliminate the type checker hack that specifically tries
AssertString. Doing so provides a 25% speedup in the
test/stdlib/ArrayNew.swift test (which is type-checker bound).

The specific simplifications here:
  - User-level
  assert/precondition/preconditionalFailure/assertionFailer/fatalError
  always take an autoclosure producing a String, eliminating the need
  for the StaticString/AssertString dance.
  - Standard-library internal _precondition/_sanityCheck/etc. always
  take a StaticString. When we want to improve the diagnostics in the
  standard library, we can provide a separate overload or
  differently-named function.
  - Remove AssertString, AssertStringType, StaticStringType, which are
  no longer used or needed
  - Remove the AssertString hack from the compiler
  - Remove the "BooleanType" overloads of these functions, because
  their usefuless left when we stopped making optional types conform
  to BooleanType (sorry, should have been a separate patch).



Swift SVN r22139

stdlib/core/String.swift

@@ -458,7 +458,12 @@ extension String : CollectionType {
 
   @availability(*, unavailable, message="cannot subscript String with an Int")
   public subscript(i: Int) -> Character {
-    fatalError("cannot subscript String with an Int")
+    _fatalErrorMessage(
+      "fatal error", 
+      "cannot subscript String with an Int",
+      __FILE__, 
+      __LINE__
+    )
   }
 
   public func generate() -> IndexingGenerator<String> {