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.
We've had these for a while, used for interface specialisation, but
they're not yet used anywhere else or properly documented. We should
document them soon, but for now, it's a useful performance boost to
always use the fast versions of pack/unpack/concat at runtime.
Also moves a couple to the prelude, to ensure that the fast versions are
defined in the same place as the 'normal' version so that the
transformation will always fire (that is, no need to import Data.String
for the transformation to work).
