Add support for static allocation of cells, strings and stringbufs.

Add support for static allocation of cells, strings and stringbufs.

* libguile/__scm.h (SCM_ALIGNED): New macro.

* libguile/_scm.h: Include "libguile/strings.h", to make the
  string/stringbuf-related constants visible to snarffed code.

* libguile/snarf.h (SCM_SUPPORT_STATIC_ALLOCATION): New macro.
  (SCM_SYMBOL, SCM_GLOBAL_SYMBOL)[SCM_SUPPORT_STATIC_ALLOCATION]: New
  alternative versions of these macros with support for (almost) static
  allocation via the use of `string->symbol'.
  (SCM_IMMUTABLE_DOUBLE_CELL, SCM_IMMUTABLE_STRINGBUF,
  SCM_IMMUTABLE_STRING): New macros.

* libguile/tags.h (SCM)[SCM_DEBUG_TYPING_STRICTNESS==1]: Use a pointer
  type that is compatible with other pointer types, to avoid potential
  violation of strict aliasing rules.