Hidden virtual table columns and table-valued functions

[trueqbit]

SQLite features Hidden columns in virtual tables, which then can be used as Table-valued functions.

This PR brings:

• Table references for C++17 in order to facilitate table-valued expressions.
• Support for eponymous virtual tables with hidden columns.
• Support for eponymous virtual tables with hidden columns as table-valued functions.
• Support for the "generate_series" extension.

Additionally:

• Unit tests for mapped_type_proxy.

Examples in sqlite_orm speak are:

dbstat

Aggregated database statistics:

storage.select(asterisk<dbstat_table>(), from(dbstat_table("main", true)));

generate series

Generate six random numbers:

storage.select(random(), from(generate_series_table(1, 6)))

Where the equivalent by explicitly using hidden columns is:

storage.select(random(), where(generate_series_table->*&generate_series::hidden::start == 1 and
                               generate_series_table->*&generate_series::hidden::stop == 6);

fts5

This is of course available for the fts5 module and its hidden rank column as well:

storage.get_all<post_table>(where(match<post_table>("fts5")), order_by(post_table->*&fts5::hidden::rank));