[Refactor][VM] Port memory_alloc to c++

[zhiics]

This PR ports the memory manifest pass to C++. This would help reduce compilation time for large models.

@masahi @icemelon9 @anijain2305 @jroesch @mbrookhart @kevinthesun

[masahi]

Do we want to remove the python version?

[zhiics]

@masahi oops, I missed a commit. We should be able to remove it

[masahi]

@zhiics FYI compiling MaskRCNN with C++ ManifestAlloc generates the following warning that I didn't see in the python version. I don't think it matters, just to let you know if you have a clue.

[11:50:01] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:01] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:01] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:04] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:04] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:04] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:07] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:07] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used
[11:50:07] /home/masa/projects/dev/tvm/src/relay/transforms/let_list.h:54: letlist not used

[zhiics]

@masahi yeah, thanks for reminding. Python pass wouldn't have it because it uses the Python ScopeBuilder. I tried a bit to see why where a LetList was deconstructed but I am not quite sure.

[masahi]

Looks great, thanks!

[masahi]

thanks @zhiics