emit error when proc is defined multiple times

[willox]

#179 related.

Here's the code I used for tgstation/tgstation#50812. A lot of errors for this still trigger in the TG codebase.

This is pretty much the only Rust I have ever written so you'll want to give it a careful look. The duplicate var definitions stuff I wrote is still messy so that's not here yet.

The test examples pretty much show what it does:

Code that works

/mob/proc/test()
    set SpacemanDMM_can_be_redefined = 1
    return

/mob/test()
    return

Code that errors

/mob/proc/test()
    return

/mob/test()
    return

<file>, line 4, column 10:
error: redefining proc /mob/test
- 1:15: previous definition is here