Display class subclasses

At the moment it's pretty basic, any idea how I could make this more useful?

Example:

```
> subclasses <environment-object>
  <warning-object>
  <dylan-object>
  <application>
  <compiler-database>
  <project-object>
  <application-object>
  <compiler-object>
  <environment-options>
  <environment-object-with-library>
  <environment-object-with-id>
```

specify depth:

```
> subclasses <environment-object> depth 2
  <warning-object>
    <project-warning-object>
    <compiler-warning-object>
      <compiler-error-object>
      <serious-compiler-warning-object>
  <dylan-object>
    <definition-object>
      <type-object>
      <domain-object>
      <dylan-function-object>
      <constant-object>
      <module-variable-object>
      <macro-object>
      <namespace-object>
    <dylan-compiler-object>
      <expression-object>
    <dylan-application-object>
      <collection-object>
      <symbol-object>
      <immediate-application-object>
  <application>
  <compiler-database>
    <dfmc-database>
  <project-object>
    <native-project-object>
      <dfmc-project-object>
  <application-object>
    <restart-object>
    <stack-frame-object>
    <breakpoint-object>
      <source-location-breakpoint-object>
      <environment-object-breakpoint-object>
    <thread-object>
    <dylan-application-object>
      <collection-object>
      <symbol-object>
      <immediate-application-object>
    <component-object>
    <register-object>
    <address-object>
    <composite-object>
      <collection-object>
      <user-object>
    <application-and-compiler-object>
      <slot-object>
      <type-object>
      <dylan-function-object>
      <variable-object>
      <source-form-object>
    <unbound-object>
    <application-code-object>
      <function-object>
      <source-form-object>
      <foreign-object>
  <compiler-object>
    <compiler-warning-object>
      <compiler-error-object>
      <serious-compiler-warning-object>
    <dylan-compiler-object>
      <expression-object>
    <name-object>
      <binding-name-object>
      <module-name-object>
    <application-and-compiler-object>
      <slot-object>
      <type-object>
      <dylan-function-object>
      <variable-object>
      <source-form-object>
  <environment-options>
  <environment-object-with-library>
    <compiler-warning-object>
      <compiler-error-object>
      <serious-compiler-warning-object>
  <environment-object-with-id>
    <definition-object>
      <type-object>
      <domain-object>
      <dylan-function-object>
      <constant-object>
      <module-variable-object>
      <macro-object>
      <namespace-object>
    <source-form-object>
      <top-level-expression-object>
      <macro-call-object>
    <user-object>
      <duim-object>
      <condition-object>
      <pair-object>
      <range-object>
      <internal-object>
```
One option (in a different PR) would be to add "..." after classes that have already been displayed rather than displaying their subclasses again.