[FIREmitter] Add support for multibit_mux

The FIR emitter currently does not support the `firrtl.multibit_mux` operation:
```mlir
firrtl.circuit "Foo" {
firrtl.module @Foo(
in %s: !firrtl.uint<1>,
in %a: !firrtl.uint<42>,
in %b: !firrtl.uint<42>
) {
%0 = firrtl.multibit_mux %s, %a, %b : !firrtl.uint<1>, !firrtl.uint<42>
%n = firrtl.node %0 : !firrtl.uint<42>
}
}
```
run through `circt-translate --export-firrtl` yields:
```
error: 'firrtl.multibit_mux' op not supported as expression
```
This can probably be implemented pretty easily by emitting these as trees of two-way multiplexers, or an array index operation.

Start with the FIR emitter expression-handling path exercised by circt-translate --export-firrtl, using the provided firrtl.multibit_mux example to reproduce the unsupported-operation error. Implement support for the operation and verify that the example translates without that error, either as a tree of two-way multiplexers or an array index operation.

Written by the indexing model from the issue text.