On #2224 @zygoloid pointed out we needed --implicit-check-not to ensure we were correctly matching output. This is the standard way we're writing explorer tests, so I was looking at unifying our lit approaches.
This is one take on it, making more use of substitutions to bring various testing into alignment, as well as symlinks to avoid config skew (maybe I'll eventually figure out a better solution than symlinks).
Makes a couple small fixes in explorer to remove end-of-line whitespace on output.
The most significant change here is that explorer now uses the chosen spelling
rather than the old `Bool` spelling. Also update a few documentation examples
and some skeletal design docs to use the chosen spelling.
Support is added for all of the non-type expression contexts where we currently accept built-in conversions, such as reordering the fields in a struct.