Calling .pad on Bool results in requirement failure with no explanation

**Type of issue**: Bug Report

**Please provide the steps to reproduce the problem:**

Run the following Scala-CLI
```scala
//> using scala "2.13.12"
//> using dep "org.chipsalliance::chisel:6.4.0"
//> using plugin "org.chipsalliance:::chisel-plugin:6.4.0"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
// _root_ disambiguates from package chisel3.util.circt if user imports chisel3.util._
import _root_.circt.stage.ChiselStage

class Foo extends Module {
val in = IO(Input(Bool()))
val out = IO(Output(UInt(8.W)))

out := in.pad(8)
}

object Main extends App {
println(
ChiselStage.emitSystemVerilog(
gen = new Foo,
firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info")
)
)
}
```

**What is the current behavior?**

You will get the following exception:
```
Exception in thread "main" java.lang.IllegalArgumentException: requirement failed
at ... ()
at Foo.$anonfun$new$1(chisel-example.scala:14)
at chisel3.Data.$anonfun$$colon$eq$1(Data.scala:754)
at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
at chisel3.experimental.prefix$.apply(prefix.scala:33)
at chisel3.Data.$colon$eq(Data.scala:754)
at Foo.(chisel-example.scala:14)
at Main$.$anonfun$new$2(chisel-example.scala:20)
at chisel3.Module$.evaluate(Module.scala:88)
at chisel3.Module$.do_apply(Module.scala:35)
at chisel3.stage.phases.Elaborate.$anonfun$transform$2(Elaborate.scala:52)
at chisel3.internal.Builder$.$anonfun$buildImpl$1(Builder.scala:1018)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
at chisel3.internal.Builder$.buildImpl(Builder.scala:1012)
at chisel3.internal.Builder$.$anonfun$build$1(Builder.scala:1003)
at logger.Logger$.$anonfun$makeScope$4(Logger.scala:148)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
at logger.Logger$.makeScope(Logger.scala:146)
at logger.Logger$.makeScope(Logger.scala:133)
at ... ()
at ... (Stack trace trimmed to user code only. Rerun with --full-stacktrace to see the full stack trace)
```

**What is the expected behavior?**

At a bare minimum, this should give a decent error message.

In an ideal world, `Bool` (a logical type) would not extend `UInt` (an arithmetic type), because I would think `pad` should not be defined on `Bool` at all. With the inheritance relationship, calling `.pad` on a `Bool` probably should return a `UInt` of the padded width, **however**, `.pad` in Chisel is defined to return `this.type` so `Bool.pad` returns `Bool` (`.this.type`). I suspect this was a mistake but it does imply that attemping to `pad` a `Bool` beyond 1-bit is illegal.

So in my opinion, the least bad option is to error, but with a reasonable message.

**Please tell us about your environment:**

**Other Information**

**What is the use case for changing the behavior?**