RFC: add a warning against having multiple copies of a shared library loaded

this is the first part in a series of improvements I would like to make to improve dealing with binary dependency (and inspired by JuliaCon!)

this might be useful because it is undefined which library C may use to lookup various functions. this can be problematic if there is global state in the shared library which is expected to be common across calls to the library from various dependencies

this behavior should probably also be part of `get_library` in `ccall.cpp` to be truly useful

the main open question is whether this should only warn about explicitly loaded dependencies, or all (as it does now, and is, I think, preferable)