program statements.

This patch contains the following refactorings:

  - Introduce enum_sp_type enum with type-code constants for stored
    procedures, functions and triggers (SP_TYPE_FUNCTION,
    SP_TYPE_PROCEDURE, SP_TYPE_TRIGGER). Those constants should be used
    instead of TYPE_ENUM_ macros.

  - Introduce sp_start_parsing() and sp_finish_parsing() functions to be
    used in the parser instead of the typical sequence of operations
    (including reset_thd_mem_root() and restore_thd_mem_root()).

  - Introduce sp_parser_data class -- the class that contains attributes
    used at the SP-parsing time only. sp_parser_data has been extracted
    from sp_head.

  - Get rid of sp_head::m_tmp_query -- a pointer, which pointed to
    different things. Introduced various push_xxx_ptr() / pop_xxx_ptr()
    instead.

  - Move LEX::trg_table_fields to sp_head::m_trg_table_fields.

  - Move LEX::trg_chistics to sp_head::m_trg_chistics.

  - Use Table_triggers_list::trigger_table in a member function
    (Table_triggers_list::prepare_record1_accessors) instead of passing
    the table object by argument.

  - Change ambiguous names
    - THD::spcont -> THD::sp_runtime_ctx
    - LEX::spcont -> LEX::sp_current_parsing_ctx
    - sp_head::m_pcont -> sp_head::m_parsing_ctx
    - sp_head::get_parse_ctx() -> sp_head::get_parsing_ctx()
    - sp_head::m_parsing_ctx -> sp_head::m_root_parsing_ctx
    - sp_head::push_backpatch() -> sp_parser_data::add_backpatch_entry()
    - sp_head::push_cont_backpatch() -> sp_parser_data::add_cont_backpatch_entry()
    - sp_head::backpatch() -> sp_parser_data::do_backpatch()
    - bp_t -> Backpatch_info
    - bp_t::lab -> Backpatch_info::label
    - sp_instr::is_marked() -> sp_instr::opt_is_marked()

  - Make LEX::sp_current_parsing_ctx private and provide get/set accessors
    for it.

  - Remove sp_runtime_ctx_saved from sp_head::execute() as it is not used.

  - Combine sp_pcontext::register_case_expr() and
    sp_pcontext::push_case_expr_id().

  - Add two new methods in the Dynamic_array template:
    - elements(uint) to directly assign the number of elements;
    - set(index, element) to directly assign item at the given index.

  - Change sp_head::m_instr to be of Dynamic_array instead of
    DYNAMIC_ARRAY.

  - Make operations return bool instead of int, use boolean values instead
    of numbers.

  - Avoid SP-variable substitution for SET-statements. It did not work
    properly anyway -- the positions of Item_splocal were not properly
    calculated.  In any case, it does not make sense to do the substitution
    for SET-statememnts, because they are not bin-logged.

  - Use placement new explicitly for SP-instruction instantiations.

  - Use accessors (sp_head::get_ip()) instead of attibutes (sp_head::m_ip).

  - Move global functions out of sp_head.h/.cc to sp.h/.cc

  - Introduce sp_printable interface to be used in the Optimizer Traces
    (to avoid using sp_instr there).

  - Introduce sp_branch_instr class -- an interface for all SP-instructions
    with destinations that need to be updated by the SP-optimizer.

  - Introduce sp_lex_instr class -- base class for SP-instructions, whose
    execution is based on expression evaluation.