Defines methods into `TraceStream` for adding line prefixes. Instead of
directly using string literals.
Example usage,
```
trace_stream_->Start() << "declaring ... " << ... ;
```
will result in,
```
->> declaring ...
```
Allow interleaving of STDOUT and STDERR check lines. Put STDOUT lines
after the line they're attached to, and STDERR lines before. If no
STDOUT check line is attached to any line, then put them all at the end
of the file instead.
This is intended to better handle the case where stdout contains
unreplaced mentions of line numbers, and also reflects that stdout is
typically a consequence of the test rather than commentary on it, so
placing it after the test seems likely to read better.
This migrates explorer tests to file_test, using the new --autoupdate
functionality. Per discussion, the trace tests that were using "not"
output are mostly migrated to checking full output. The main exception
is tests that were including trace output from the prelude: the prelude
output is pretty long (multiple MB already) and I think it wasn't the
intent to include, only trace output from the small program.
This is the remaining use of lit support, so the supporting libraries
are also removed here.
Prabhat Sachdeva
Richard Smith
Jon Ross-Perkins