Add compile warning for "confusable" types

We have a number of "types" like integer which are not actually
supported as builtin types -- instead they are silently interpreted
as class types.

I've seen this cause confusion a few types already. This change adds
a warning in this case. In the unlikely case that someone legitimately
wants to type against an integer class, the warning can be suppressed
by writing \integer or "use integer", or using Integer (this warning
will only trigger for lowercase spellings).

Closes GH-4815.

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