Proposal: Warn for #includes from which no symbols are actually used

[ghost]

In a similar vein to #234 (which I would have just amended with this but there's already #235 that closes it 😬), another warning for improper use of #include.

A new warning, -Wunused-include, triggers when there is an #include from which no symbols are used. This reduces unnecessary dependencies and the number of files to be reanalysed during incremental compilation¹.

Footnotes

1. https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md ↩

[ghost]

Oh. Oh wow.
Building Janet with this presents 215 warnings, of which 7 are from the project itself and maybe one is a true positive.

This was a bad idea.