This is the prerequisite patch to move the sp_instr class and classes derived
from it into the files sp_instr.cc/sp_instr.h. The classes sp_lex_cursor and
sp_lex_keeper are also moved to the files files sp_instr.cc/sp_instr.h.

Additionally,
  * all occurrences of macroses NULL, FALSE, TRUE are replaced
    with the corresponding C++ keywords nullptr, false, true.
  * the keyword 'override' is added in and the keyword 'virtual' is removed
    from signatures of every virtual method implemented in classes derived
    from the base class sp_instr.
  * the keyword 'final' is added into declaration of the class sp_lex_keeper
    since this class shouldn't have a derived class by design.
  * the function cmp_rqp_locations is made static since it is not called
    outside the file sp_instr.cc.
  * the function subst_spvars() is moved into the file sp_instr.cc since this
    function used only by the method sp_instr_stmt::execute