Redesign and harden string interning.

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

src/Makefile

@@ -132,7 +132,6 @@ XCFLAGS=
 #
 # This define is required to run LuaJIT under Valgrind. The Valgrind
 # header files must be installed. You should enable debug information, too.
-# Use --suppressions=lj.supp to avoid some false positives.
 #XCFLAGS+= -DLUAJIT_USE_VALGRIND
 #
 # This is the client for the GDB JIT API. GDB 7.0 or higher is required