Core: add references handling macros

* lasso/utils.h:
  - add macro lasso_transfer_string and lasso_transfer_gobject,
    to transfer ownership of such objects without copying or
    their changing reference count. The old containing variable is
    NULLed.
  - lasso_list_add_gobject,lasso_list_add_new_gobject: test if the added
    object is a GObject, if not do not add it and print a warning.
  - lasso_check_type_equality: this macro use special builtin functions
    only with GCC (typeof and __builtin_types_compatible_p) and do
    metaprogramming using anonymous enumeration type to make compile
    time assertions. It is used
 -  add macros to release XPathObject XPathContext, macro constructor to
    make reference transfert macros (free dest, nullify dest, copy src
    to dest without increasing refcount, nullify src), add a macro to
    transfert xpath objects.

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