Better error checking

[jarcane]

Developing in Heresy at the moment is a rather gigantic pain in the ass, largely because few of our functions have proper checks for bad inputs, and this gets further obfuscated by some of the levels of default abstraction now layered onto things.

I'm finding that a lot of trivial errors in particular don't get reported in their actual places, but somehow manage to throw strange errors in #%app way back in base.rkt, which is terribly unhelpful and often gives the mistaken impression of bugs in the main implementation.

For instance, calling slice$ with an out of index start point gives this rather unhelpful reply:

> (slice$ "[++--[+]]+" 0 8)
. . ..\private\base.rkt:397:31: cdr: contract violation
  expected: pair?
  given: '()

The long hard solution to this I think is simply going to mean going through each function and adding error checks and proper error calls for at least more common errors. This task, like the documentation task, will remain a perpetually open checklist. As I find more obvious errors that should be checked, I will update the following checklist.

• splice/splice$ should check for less-than-1 index

[jarcane]

As discussed in #58, consider adding an error or at least a warning when a child thing overrides a field in the parent thing.