Automatically register FFI functions (dlsym path)

[PaulBone]

Currently anyone using the FFI must arrange for foreign function implementations to be registered with the vm. This should be automatic and provided in two ways:

• The Plasma compiler generates foreign code to do this (Automatically register FFI functions (static linking) #456 )
• The Runtime uses dlsym to find foreign functions.

This bug will implement the second. This can be deferred to a later goal.