Related to c2c8808a

* cleanup: *never* use assert(A && B)
* vers_setup_1() revisited
* vers_setup_2() renamed
* partition_element::type removed
* Copy ctor instead of memcpy()
* Handle return value from check_range_constants()
* Malloc error fix
* error, style, misc fixes

* Sys_var_vers_asof formatting
* Vers_field_stats renamed to Vers_min_max_stats
* Item_temporal_literal::set_lower()/set_higher() replaced by operator>()/operator<()
* handler API comments

[closes #211]

Due to standard (see 7.6 <table reference>).

Eliminated `QUERY FOR`.