Fix calls to GC.addRange and GC.removeRange to be const-correct in root/rmem.d

[JinShil]

In support of dlang/druntime#2706

The interface to GC.addRange and GC.removeRange is not const

https://github.com/dlang/druntime/blob/a370406fae72602ab2e0fe632d7a70a71da79267/src/gc/proxy.d#L228-L231

https://github.com/dlang/druntime/blob/a370406fae72602ab2e0fe632d7a70a71da79267/src/gc/proxy.d#L238-L241

https://github.com/dlang/druntime/blob/a370406fae72602ab2e0fe632d7a70a71da79267/src/gc/impl/proto/gc.d#L27-L28

https://github.com/dlang/druntime/blob/a370406fae72602ab2e0fe632d7a70a71da79267/src/gc/impl/proto/gc.d#L202

https://github.com/dlang/druntime/blob/a370406fae72602ab2e0fe632d7a70a71da79267/src/gc/impl/proto/gc.d#L207

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[JinShil]

See also dlang/druntime#2704 (review)

[rainers]

I think this should not be necessary, see dlang/druntime#2706 (comment)

[JinShil]

Closing. There are some const-ness problems in the GC interface and implementations, but fixing that will have to be a different PR.