Fix issue #665: Make EasyKernel more configurable

Escape html control characters in error messages.

Otherwise these are misinterpreted as MathJax again (even in their `$` form, for whatever reason!).

Error messages are usually rendered surprisingly clean even without escaping, but in a few cases it went wrong quite badly, in particular the `<<loop>>` error would get rendered as the completely cryptic `<>`.

Also, something like `"one string&quot; &quot;another string" 4` used to give the error message

```
The function ‘"one string" "another string"’ is applied to one argument,
but its type ‘String’ has none
```

This kind of behavior is prevented by escaping the characters `<`, `>` and `&` to `&lt;`, `&gt;` and `&amp;`, respectively.

This fix is only a halfway-fix. If there are multiple IHaskell libraries
installed, you can still end up loading an interpreted version of the
wrong one; however, now Display values are serialized to bytestring, so
this will not cause any issues with transferring those between the two
versions. Widget messages are not serializable and cannot be
serializable, so those remain a problem, but instead of crashing it will
instead emit a loud warning on stderr.

This should make the build faster and more reliable,
and hopefully ressurect the test suite.

Also, add some utilities for working with IHaskell components in `stack
ghci`.