Make identifiers str (not str8) objects throughout.
This affects the parser, various object implementations,
and all places that put identifiers into C string literals.

In testing, a number of crashes occurred as code would
fail when the recursion limit was reached (such as the
Unicode interning dictionary having key/value pairs where
key is not value). To solve these, I added an overflowed
flag, which allows for 50 more recursions after the
limit was reached and the exception was raised, and
a recursion_critical flag, which indicates that recursion
absolutely must be allowed, i.e. that a certain call
must not cause a stack overflow exception.

There are still some places where both str and str8 are
accepted as identifiers; these should eventually be
removed.
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index da8af34..f660046 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -515,7 +515,7 @@
             /* Hangul Decomposition adds three characters in
                a single step, so we need atleast that much room. */
             if (space < 3) {
-                Py_ssize_t newsize = PyString_GET_SIZE(result) + 10;
+                Py_ssize_t newsize = PyUnicode_GET_SIZE(result) + 10;
                 space += 10;
                 if (PyUnicode_Resize(&result, newsize) == -1)
                     return NULL;