Add a forward declaration header.

[jzmaddock]

From ML question. Seems reasonable.

[libbooze]

Hi @jzmaddock thank you for opening the issue, here is how I have imagined it being used, lmk if that sounds implausible. If it does not great. :)

[shardator]

This would be very useful, some boost headers pull in a huge amount of code. For large projects this can increase compile times.