I recently learned that the typical use of libsecp contexts, in non-hot-path situations like during app startup of a heavyweight app, is just to give all the flags all the time.

I think it would be slightly helpful to document this by creating a SECP256K1_CONTEXT_ALL flag, and commenting it with a note on when it's appropriate to use (i.e. when a few dozen milliseconds aren't a big deal.)

CC: @apoelstra from earlier discussion