util.SteamIDToAccountID and util.SteamIDFromAccountID

[mastopgunaf]

Added functions to convert between SteamID and AccountID (aka SteamID3)

[Kefta]

This will be added in the engine if it is at all so we can also add ID64 conversion functions.

[robotboy655]

I am going to deny this request solely based on the fact that AccountID should be avoided altogether. It contains less information that a STEAM_0:0:0 SteamID and theoretically could have collisions.

You should be using the 64bit SteamIDs.

See this page for more information: https://developer.valvesoftware.com/wiki/SteamID