Consider using version-scoped namespaces to maintain backwards-compatibility

[miraclx]

The WASM<->VM ABI should be stable enough to have old wasm blobs deployed on the registry at whatever point "just work" with any node at any point in time.

To do this, the node has to maintain all of the old/deprecated interfaces behind special namespaces.

This is already possible with the attribute:

#[link(wasm_import_module = "env")]

env is the default, and is what we support now. But we should use custom ones and scope out those breaking changes under them.

[miraclx]

This is also useful to prevent treating those host functions from being reserved when #[app::logic] methods are #[no_mangle]-d and exposed into the same symbol scope as the sys ones.

A reduced scope of this is required by and will be handled in #368.