Add more functions:
- Numeric Functions: PI(),ROUND(),TRUNC(),EXP(),LN(),SIGN(),DEGREES(),RADIANS().
- String Functions: CHAR(),ASCII(),POSITION(),TRIM(),REPLACE(),REPEAT().
- Date and Time Functions: WEEKDAY(),DAYOFWEEK(),WEEK(),WEEKOFYEAR().
- Aggregate Functions: VAR_POP().
Modify some functions:
- ROUND(): support round to given decimal places.
- CHAR_LENGTH(): support calculate multi-byte character's length.
- TIMEDIFF(): result will be negative when expr1 < expr2.
- STDDEV(): add alias name STDDEV_POP().
- SUBSTR(): add alias name SUBSTRING().
support new syntax SUBSTRING/SUBSTR(expr FROM pos [FOR len]).
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