Add method 'exists' to core/shm

[dpino]

Trying to open a shared memory object which has not been mapped yet results in an error. For instance:

core/shm.lua:37: shm open error (26039/shm/selftest/obj):No such file or directory

I think it would be useful to have an exists method which could be used to check whether an object exists before trying to open it.

[eugeneia]

I have previously done this by using shm.children and probing the resulting table. Alternatively we could use pcall to catch the error, but that would be less granular. Obviously convenient, merged (I will edit the documentation a bit to describe the return type). 👍