maint: Fix autoreconf call in bootstrap script

* bootstrap: Likewise.
maint: Fix 'pointerof' definition; add 'cast' macro

* macros.h: Likewise.
maint: Add mregistry macros to luax-macros.h

* luax-macros.h (luax_mregistry_create): New macro; creates a local
registry.
(luax_mregistry_destroy): New macro; destroys local registry.
(luax_mregistry_get): New macro; pushes local registry onto stack.
(luax_mregistry_getfield): New macro; gets field from local registry.
(luax_mregistry_setfield): New macro; sets local registry field.