Add variable-list or function to determine or list required features...

[srcshelton]

It would be helpful to be able to query a programmatic reference to specify the features needed, or to find out when a feature was first implemented (without requiring repo history).

For example, std::firstseen var std_LASTOUTPUT might return 2.0.4 whilst std::firstseen func std::vcmp could return 1.4.

(std::firstversion or std::initialversion, or even std::feature might be a better name?)

Alternatively, the same data could simply be exposed as std_VAR and std_FUNC associative arrays, linking name and release. More generically, std_FEATURES could link both features to versions, and versions to a list of features (where a 'feature' is an external variable or function).

std::requires could potentially then be updated to also support requiring specified internal facilities, as well as external tools...