Redesign and harden string interning.

Up to 40% faster on hash-intensive benchmarks.
With some ideas from Sokolov Yura.

src/lj_arch.h

@@ -637,14 +637,31 @@ extern void *LJ_WIN_LOADLIBA(const char *path);
 
 /* Don't make any changes here. Instead build with:
 **   make "XCFLAGS=-DLUAJIT_SECURITY_flag=value"
+**
+** Important note to distro maintainers: DO NOT change the defaults for a
+** regular distro build -- neither upwards, nor downwards!
+** These build-time configurable security flags are intended for embedders
+** who may have specific needs wrt. security vs. performance.
 */
 
 /* Security defaults. */
 #ifndef LUAJIT_SECURITY_PRNG
+/* PRNG init: 0 = fixed/insecure, 1 = secure from OS. */
 #define LUAJIT_SECURITY_PRNG	1
 #endif
 
+#ifndef LUAJIT_SECURITY_STRHASH
+/* String hash: 0 = sparse only, 1 = sparse + dense. */
+#define LUAJIT_SECURITY_STRHASH	1
+#endif
+
+#ifndef LUAJIT_SECURITY_STRID
+/* String IDs: 0 = linear, 1 = reseed < 255, 2 = reseed < 15, 3 = random. */
+#define LUAJIT_SECURITY_STRID	1
+#endif
+
 #ifndef LUAJIT_SECURITY_MCODE
+/* Machine code page protection: 0 = insecure RWX, 1 = secure RW^X. */
 #define LUAJIT_SECURITY_MCODE	1
 #endif