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/replace.inc

@@ -0,0 +1,54 @@
+/* clang-format off */
+/* stringlib: replace implementation */
+
+#ifndef STRINGLIB_FASTSEARCH_H
+#error must include fastsearch.inc before including this module
+#endif
+
+Py_LOCAL_INLINE(void)
+STRINGLIB(replace_1char_inplace)(STRINGLIB_CHAR* s, STRINGLIB_CHAR* end,
+                                 Py_UCS4 u1, Py_UCS4 u2, Py_ssize_t maxcount)
+{
+    *s = u2;
+    while (--maxcount && ++s != end) {
+        /* Find the next character to be replaced.
+
+           If it occurs often, it is faster to scan for it using an inline
+           loop.  If it occurs seldom, it is faster to scan for it using a
+           function call; the overhead of the function call is amortized
+           across the many characters that call covers.  We start with an
+           inline loop and use a heuristic to determine whether to fall back
+           to a function call. */
+        if (*s != u1) {
+            int attempts = 10;
+            /* search u1 in a dummy loop */
+            while (1) {
+                if (++s == end)
+                    return;
+                if (*s == u1)
+                    break;
+                if (!--attempts) {
+                    /* if u1 was not found for attempts iterations,
+                       use FASTSEARCH() or memchr() */
+#if STRINGLIB_SIZEOF_CHAR == 1
+                    s++;
+                    s = memchr(s, u1, end - s);
+                    if (s == NULL)
+                        return;
+#else
+                    Py_ssize_t i;
+                    STRINGLIB_CHAR ch1 = (STRINGLIB_CHAR) u1;
+                    s++;
+                    i = FASTSEARCH(s, end - s, &ch1, 1, 0, FAST_SEARCH);
+                    if (i < 0)
+                        return;
+                    s += i;
+#endif
+                    /* restart the dummy loop */
+                    break;
+                }
+            }
+        }
+        *s = u2;
+    }
+}