time.Time is now supported as a column type

(1) gotables now supports time.Time as a column type

RFC 3339 time literals are accepted as cell values and produced as string output.
[TimeTable]
t0 time.Time = 2020-03-15T14:22:30Z
t1 time.Time = 2020-03-15T14:22:30+17:00
t2 time.Time = 2020-03-15T14:22:30-17:00
t3 time.Time = 2020-03-15T14:22:30.12345Z
t4 time.Time = 2020-03-15T14:22:30.12345+17:00
t5 time.Time = 2020-03-15T14:22:30.12345-17:00

gotables.MinTime and gotables.MaxTime are the earliest and latest times that
a time.Time variable can represent.

(2) JSON now supports nested tables and time.Time column types.

A check is done during JSON marshalling to prevent circular references.

table.IsValidTableNesting() checks for potential circular references.

(3) GOB support has been reimplemented. But for some reason bench tests
indicate this implementation is slow. Table nesting and time.Time are not
supported.

(4) A *gotables.TableSet string literal can now include a TableSet name in the form:
[[MyTableSetName]]

The [[...]] syntax can occur before any table in the TableSet, but ideally before the
first table.