Manage memory residency of tensors

[geekbeast]

Support multi-device memory residency of tensors.

[abrown]

Like we discussed (and as commented in #70), this change is probably not desirable if we adopt https://github.com/WebAssembly/wasi-nn/pull/74: if a user can't change where a model resides any more, they shouldn't be allowed to change where a tensor resides.