Rename edge to slot (backport #150)

[mergify]

Parent PR: mmtk/mmtk-core#1134

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This is an automatic backport of pull request #150 done by Mergify.

[mergify]

Cherry-pick of e776c41 has failed:

On branch mergify/bp/v1.9.2+RAI/pr-150
Your branch is up to date with 'origin/v1.9.2+RAI'.

You are currently cherry-picking commit e776c41.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   julia/mmtk_julia.c
	modified:   mmtk/api/mmtk.h
	deleted:    mmtk/src/edges.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   mmtk/Cargo.lock
	both modified:   mmtk/Cargo.toml
	both modified:   mmtk/src/api.rs
	both modified:   mmtk/src/julia_scanning.rs
	both modified:   mmtk/src/lib.rs
	both modified:   mmtk/src/scanning.rs
	both modified:   mmtk/src/slots.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[qinsoon]

We should back port #146 first.