Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Objects/stringlib/localeutil.inc

@@ -0,0 +1,83 @@
+/* clang-format off */
+/* _PyUnicode_InsertThousandsGrouping() helper functions */
+
+typedef struct {
+    const char *grouping;
+    char previous;
+    Py_ssize_t i; /* Where we're currently pointing in grouping. */
+} GroupGenerator;
+
+
+static void
+GroupGenerator_init(GroupGenerator *self, const char *grouping)
+{
+    self->grouping = grouping;
+    self->i = 0;
+    self->previous = 0;
+}
+
+
+/* Returns the next grouping, or 0 to signify end. */
+static Py_ssize_t
+GroupGenerator_next(GroupGenerator *self)
+{
+    /* Note that we don't really do much error checking here. If a
+       grouping string contains just CHAR_MAX, for example, then just
+       terminate the generator. That shouldn't happen, but at least we
+       fail gracefully. */
+    switch (self->grouping[self->i]) {
+    case 0:
+        return self->previous;
+    case CHAR_MAX:
+        /* Stop the generator. */
+        return 0;
+    default: {
+        char ch = self->grouping[self->i];
+        self->previous = ch;
+        self->i++;
+        return (Py_ssize_t)ch;
+    }
+    }
+}
+
+
+/* Fill in some digits, leading zeros, and thousands separator. All
+   are optional, depending on when we're called. */
+static void
+InsertThousandsGrouping_fill(_PyUnicodeWriter *writer, Py_ssize_t *buffer_pos,
+                             PyObject *digits, Py_ssize_t *digits_pos,
+                             Py_ssize_t n_chars, Py_ssize_t n_zeros,
+                             PyObject *thousands_sep, Py_ssize_t thousands_sep_len,
+                             Py_UCS4 *maxchar)
+{
+    if (!writer) {
+        /* if maxchar > 127, maxchar is already set */
+        if (*maxchar == 127 && thousands_sep) {
+            Py_UCS4 maxchar2 = PyUnicode_MAX_CHAR_VALUE(thousands_sep);
+            *maxchar = Py_MAX(*maxchar, maxchar2);
+        }
+        return;
+    }
+
+    if (thousands_sep) {
+        *buffer_pos -= thousands_sep_len;
+
+        /* Copy the thousands_sep chars into the buffer. */
+        _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
+                                      thousands_sep, 0,
+                                      thousands_sep_len);
+    }
+
+    *buffer_pos -= n_chars;
+    *digits_pos -= n_chars;
+    _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
+                                  digits, *digits_pos,
+                                  n_chars);
+
+    if (n_zeros) {
+        *buffer_pos -= n_zeros;
+        enum PyUnicode_Kind kind = PyUnicode_KIND(writer->buffer);
+        void *data = PyUnicode_DATA(writer->buffer);
+        FILL(kind, data, '0', *buffer_pos, n_zeros);
+    }
+}