RValue as const Eq parsing failure: unexpected Eq

[yanliu18]

On branch builtin-identifiers, the handwritten test- https://github.com/runtimeverification/mir-semantics/blob/refactor/builtin-identifiers/kmir/src/tests/integration/test-data/handwritten-mir/parsing/const-enum-same-as-builtin.mir
failed due to syntax error, unexpected Eq, expecting fn

It is expected to be parsed as the RValue::Use(Operand)::Constant kind.

This issue is related to

mir-semantics/kmir/k-src/mir-rvalue.md

Line 160 in b5be2bb

TODO: these sorts may need refactoring to closer match the `rustc` implementation.

and the issue #54

It is very likely we might need to investigate if we need to include 'tcx (type context) implementation in the semantic model.
It is also related to the problem of Rust's memory model #170