Implement link-time garbage collection

[bitwalker]

The implementation of the linker in #24 left a few things undone because they aren't critical to an MVP release. Namely, garbage collection of unused (dead) objects discovered at link-time.

Items To Garbage Collect

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1. Functions with external linkage which are never called¹
2. Functions with internal linkage which are never called, or are transitively dead (all callers are dead)²
3. Globals which are never referenced¹, or are transitively dead (i.e. references are all from dead functions)
4. Modules which are dead because all of their contents are dead
5. Data segments which are dead because they were only declared in dead modules

¹ Only when the program being linked is an executable program. When linking a library, we must assume all objects with external linkage are used.
² Objects with internal (or ODR) linkage are always candidates for garbage collection if there are no references from live objects.

This task is relatively low priority, but is a relatively straightforward task to tackle when we have time.