MDEV-20022 sql_mode="oracle" does not support TO_NUMBER() function

This patch implements an Oracle style function to_number()
with the following signatures:

- to_number(number_or_string_subject)
- to_number(string_subject, string_format)

The function is implemented in sql/item_numconvfunc.cc.

The function returns the DOUBLE data type for
all signatures and input data types.

The format parser understands the following components:

- Digits: 0, 9
- Hex digits: X
- Group separators: comma (,) and G
- Decimal delimiers: period (.) and D
- Approximate number signature: EEEE
- Currency/numeric flags: $ and B
- Currency signatures: C, L, U
- Sign signatures: S, MI, PR
- Special format signatures: V, TM, TM9, TME
- Format flag: FM

Note, the parser was implemented assuming that we'll also have the
oppostite function to_char() soon for numeric input.
So it parser all known components.
However, the function to_number() does not support:
- Formats V, TM, TM9, TME. to_number() returns NULL if the format
  string has these components.
  These componens are supported only by to_char() in Oracle.

Features not inclided into this patch:
- The ON CONVERSION ERROR clause
- The third parameter (nlsparam)
- Internationalized components: G, D, C, L, U.
These features will be implemented later, under terms of MDEV-36978.

Notable changes in the other files:

- item_func.h: adding Item_handled_func::Handler_double

- simple_parser.h: adding a number of *CONTAINER* templates
  They help to save on duplicate code when creating classes
  suitable for passing into parsing templates such as OPT, OR2C, OR3C, etc

- simple_parser.h: Adding parsing templates OR4C and OR5C

- simple_parser.h: Moving the template "OPT" towars the beginning of the file
  Rule parsing templates TOKEN, TokenChoice, AND2, OR2C, OR3C, OR4C, OR5C, LIST
  now provide a sub-class Opt, to parse its optional rule.

- simple_parser.h: Adding "explicit" to all "operator bool" definitions

- Renaming Parser_templates::TOKEN to Parser_templates::TokenParser

- Adding "explicit" to all "operator bool()" templates/classes,
  to avoid hidden implicit conversion (to int, void*, etc).

- Renaming the LIST template parameter ELEMENT to ELEMENT_PARSER,
  to make it clearer what it is for.

- Renaming the OPT template parameter RULE to RULE_PARSER,
  to make it clearer what it is for.

Author: abarkov

libmysqld/CMakeLists.txt

@@ -64,6 +64,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/item.cc ../sql/item_create.cc ../sql/item_func.cc 
            ../sql/item_geofunc.cc ../sql/item_row.cc ../sql/item_strfunc.cc 
            ../sql/item_subselect.cc ../sql/item_sum.cc ../sql/item_timefunc.cc 
+           ../sql/item_numconvfunc.cc
            ../sql/item_xmlfunc.cc ../sql/item_jsonfunc.cc
            ../sql/json_schema.cc ../sql/json_schema_helper.cc
            ../sql/key.cc ../sql/lock.cc ../sql/log.cc ../sql/log_cache.cc