[FIRRTL] Allow Layer Inlining in Enabled Layers

For situations where a module has a layer enabled, inline a layerblock with a compatible layer. Consider the following basic example:

firrtl.circuit "Foo" {
  firrtl.layer @A bind {}
  firrtl.module @Foo() attributes {layers=[@A]} {
    firrtl.layerblock @A {
      %a = firrtl.wire : !firrtl.uint<1>
    }
  }
}

This can be converted to:

firrtl.circuit "Foo" {
  firrtl.layer @A bind {}
  firrtl.module @Foo() attributes {layers=[@A]} {
    %a = firrtl.wire : !firrtl.uint<1>
  }
}

This cannot likely be done with a canonicalizer and needs a pass. However, there may be MLIR infra that can be reused here as the inlining is very simple.