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.
There are a few implicit conversions that are implemented by an `impl` of `ImplicitAs` that delegates to code in explorer:
- Converting between tuple types
- Converting from tuples of types to `type`
- Converting from tuples of values to an array type
- Converting between struct types
- Converting from a struct type to a class type
These conversions can all rely on performing more conversions for elements or subobjects, but previously those inner conversions could only be performed if they were built into explorer. This change instead uses the full implicit conversion machinery in explorer to perform these conversions, including searching for a user-defined `impl` of `ImplicitAs` when necessary.
For example, this permits a conversion from `{.a: T}` to `{.a: U}`, or from `(T, T)` to `(U, U)`, or from `(T, T)` to `[U; 2]` when there is a user-defined conversion from `T` to `U`.
Depends on #2878
Richard Smith