move builtin to register, where it belongs

lib/register/builtin/README.md

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+### Builtin module
+
+The Builtin module contains functions that can not be coded in ruby. 
+It is the other side of the parfait coin, part of  the runtime.
+
+The functions are organized by their respective class and get loaded in boot_classes! , right at the start.
+
+These functions return their code, ie a Virtual::CompiledMethod object, which can then be called by ruby code 
+as if it were a "normal"  function.
+
+A normal ruby function is one that is parsed and transformed to code. But not all functionality can be written in ruby, 
+one of those chicken and egg things. C uses Assembler in this situation, we use Builtin functions.
+
+Slightly more here : http://salama.github.io/2014/06/10/more-clarity.html (then still called Kernel)
+
+The Builtin module is scattered into several files, but that is just so the file doesn't get too long.
+