## Rationale
`SizeVar` encodes nonnegativity in runtime subtype identity, which is
fragile under cloning and remapping. Symbolic integer values should use
one `Var` representation, with nonnegative facts recorded in the
analyzer at the use sites that establish them.
## Changes
- Remove `SizeVar` from the C++, Python, TE, TVMScript, FFI, visitor,
and serialization surfaces, and migrate callers to `Var`.
- Preserve the existing Relax constraint ownership model and use
`MarkGlobalNonNegValue` as the canonical path for global nonnegative
facts.
- Preserve `T.handle()` as the normal opaque-handle form. An optional
dtype constructs a typed pointer, with `T.handle("void")` reserved for
an explicit pointer-to-void.
Tianqi Chen
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