it's not enough to look for NOT NULL IS, this also fails queries like
SELECT NOT NULL <=> NULL;
and adds no value anymore, as the grammar now requires parentheses

* remove a confusing method name - Field::set_default_expression()
* remove handler::register_columns_for_write()
* rename stuff
* add asserts
* remove unlikely unlikely
* remove redundant if() conditions
* fix mark_unsupported_function() to report the most important violation
* don't scan vfield list for default values (vfields don't have defaults)
* move handling for DROP CONSTRAINT IF EXIST where it belongs
* don't protect engines from Alter_inplace_info::ALTER_ADD_CONSTRAINT
* comments

make it return same errors on CREATE as

CREATE ... ( ... DEFAULT const ... )

the error should be "subselect is not allowed here", same as for
DEFAULT ((SELECT 1))

and remove all related casts to uchar*
also remove a couple of unused methods

collaterals:
* use %`s, not '%s'
* use correct SQLSTATE codes for these two errors

- Adding SHOW CREATE TABLE into all DEFAULT tests,
  to cover need_parentheses_in_default() for all items
- Fixing a few items not to print parentheses in DEFAULT:
  spatial function-alike predicates, IS_IPV4 and IS_IPV6 functions,
  COLUMN_CHECK() and COLUMN_EXISTS().

(a new rule in sql_yacc.yy)

- Force usage of () around complex DEFAULT expressions
- Give error if DEFAULT expression contains invalid characters
- Don't use const_charset_conversion for stored Item_func_sysconf expressions
  as the result is not constaint over different executions
- Fixed Item_func_user() to not store calculated value in str_value

ENCRYPT(), SHA(), SHA2(),
AES_ENCRYPT(), AES_DECRYPT(),
DES_ENCRYPT(), DES_DECRYPT()

CRC32(), MD5(), FROM_BASE64(), TO_BASE64(), HEX(), UNHEX(),
ENCODE(), DECODE(), PASSWORD(),
COMPRESS(), UNCOMPRESS(), UNCOMPRESSED_LENGTH().

UUID(), UUID_SHORT()

Miscelaneous functions:
BENCHMARK(), SLEEP(), ROW_COUNT(), FOUND_ROWS(),
GET_LOCK(), RELEASE_LOCK(), IS_USED_LOCK(), IS_FREE_LOCK(),
MASTER_POS_WAIT(), MASTER_GTID_WAIT(), BINLOG_GTID_POS(),
ST_GIS_DEBUG(), DECODE_HISTOGRAM(),

Miscelaneous functions:
LIKE_RANGE_MIN(), LIKE_RANGE_MAX(), WEIGHT_STRING(),
GET_FORMAT(), FORMAT(), LOAD_FILE().

Metadata functions

Dynamic column functions

Functions DATABASE() and USER().

INET4/INET6 functions

- INT result functions
- CAST variants: COLLATE, BINARY, CONVERT(..USING)

UDF

Numeric functions with string input

Boolean predicates

XML functions

Adding tests for cast, bit functions, string functions.

GIS functions

Testing non-ASCII string literals.

- real functions
- temporal functions
- hybrid functions