allow modification of memory for segments not backed by files

[psifertex]

Currently, it is not possible to modify memory unless it's backed by a file. It should be possible to arbitrary memory though. Requires a new ability to store arbitrary memory in the bndb among other changes.

[joshwatson]

Yes please. I can’t believe there isn’t a ticket for this already haha. I’ve wanted this since I started implementing an LLIL emulator!

[psifertex]

Related to #1508