Add functions for MySQL

[grieve54706]

We added a test case for all functions. But don't have time to fix all. So we move out some functions that need to be revised and submit this issue to add them back.

function_type,name,return_type,param_names,param_types,description
scalar,trim,varchar,,"varchar","Removes leading and trailing spaces from a string"
scalar,ifnull,any,,"any,any","Returns the first argument if it is not NULL, otherwise returns the second argument"
scalar,case,any,,"array<boolean>,array<any>","Makes conditional statements like if-then-else"
scalar,cast,any,,"any,varchar","Converts a value from one data type to another"
scalar,md5,varchar,,"varchar","Calculates an MD5 hash value"
scalar,make_set,varchar,,"int,array<varchar>","Returns set of comma-separated strings that have corresponding bit set"
scalar,substring_index,varchar,,"varchar,varchar,int","Returns substring from string before specified number of delimiter occurs"
scalar,time_format,varchar,,"time,varchar","Formats time according to format specification"
scalar,time_to_sec,int,,"time","Converts time value to seconds"
scalar,unix_timestamp,int,,,,"Returns Unix timestamp"
scalar,date_add,datetime,,"datetime,interval","Adds time/date interval to date"
scalar,date_sub,datetime,,"datetime,interval","Subtracts time/date interval from date"
aggregate,avg,decimal,,"any","Returns the average of values in a group"
aggregate,count_distinct,int,,"any","Returns count of distinct values in group"
window,row_number,int,,,,"Returns the sequential number of a row within a partition"
window,rank,int,,,,"Returns the rank of current row within a partition, with gaps"
window,dense_rank,int,,,,"Returns the rank of current row within a partition, without gaps"
window,lag,any,,"any,int","Returns the value from a previous row in the partition"
window,lead,any,,"any,int","Returns the value from a following row in the partition"
window,first_value,any,,"any","Returns the first value in the window frame"
window,last_value,any,,"any","Returns the last value in the window frame"
window,nth_value,any,,"any,int","Returns value from the nth row of the window frame"
window,ntile,int,,"int","Divides partition into specified number of groups"
window,percent_rank,decimal,,,,"Returns relative rank of current row (rank-1)/(total rows-1)"
window,cume_dist,decimal,,,,"Returns cumulative distribution of current row"

[goldmedal]

instr is disabled by #962