Why:
* To implement robust cross-project go-to-definition in LSP
i.e you can jump to definition of any global name coming
from library dependencies, as well as from the local project files.
What it does:
* Modify `FC`s to carry `ModuleIdent` for .idr sources,
file name for .ipkg sources or nothing for interactive runs.
* Add `--install-with-src` to install the source code alongside
the ttc binaries. The source is installed into the same directory
as the corresponding ttc file. Installed sources are made read-only.
* As we install the sources pinned to the related ttc files we gain
the versioning of sources for free.
As it was, there was significant backtracking for big expressions,
getting to the end, not finding a **, so having to try again for
application expressions. Fixes#532
