break up package builds into two steps (sometimes stack leaks memory); try to forcefully apply --fast to all deps

It's a bit closer to a dictionary.

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.