Use firrtl.bitcast operator for DataView/asTypeOf conversions

Problem Statement
---

FIRRTL’s intermediate representation already defines a `BitCastOp` for bit-level reinterpreting between equal-width types (`Bundles`, `Vecs`, `UInt`, `SInt`, etc.), but this operator currently lacks parser/printer (ser/deser) support.
As a result, Chisel and CIRCT cannot leverage `BitCastOp` in emitted FIRRTL IR, falling back to DSL-side recursive split-and-reconnect logic for `asTypeOf`, `DataView`, `viewAs`, and similar conversions.

Desired Behavior
---
Enable full FIRRTL parser and printer support for BitCastOp so that Chisel can lower all type reinterpret operations directly to a single BitCastOp, eliminating the recursive split-and-reconnect pattern.
I provide a related implementation here, which I think it's really cool and reduced the eDSL maintaining burden: sequencer/zaozi#70

Proposed Solution
---
- FIRRTL Parser/AST updates in both CIRCT and chisel;
- Update implementation of `DataView` to use it;

Start by reading the existing FIRRTL BitCastOp definition and tracing parser/printer paths in CIRCT and Chisel, then inspect the DataView implementation and the related sequencer/zaozi#70 change. Done means BitCastOp can be parsed and printed and DataView conversions use it instead of recursive split-and-reconnect logic.

Written by the indexing model from the issue text.