The preserved path is exposed by a new `module-path-index-submodule'
function, and `module-path-index-join' now accepts an optional
submodule path.
Also, fixed a problem with `collapse-module-path-index' when
a module path indx is built on a resolved module path that
is a submodule path.
In addition to the main repair, `collapse-module-path[-index]' is
correctly documented to allow '(quote <sym>) rel-to paths.
Finally, `collapse-module-path-index' changed to use a symbolic
resolved module path that appears as the base of a module path
index, rather than falling back to the given rel-to path. It's
possble that the old beavior was intentional, but it wasn't tested,
and it seems more likely to have been a bug.
Closes PR 12724
Rename `read-intern-literal' to `datum-intern-literal'.
Interning is needed only in `read-syntax' or `datum->syntax' to
set up the invariants that the bytecode compiler needs for cross-module
optimization. When `read'ing numbers from a data file, meanwhile,
interning slows things down a lot and doesn't seem worthwhile.
This change saves a small amount of space in cross-reference files
and some space in loaded cross-reference information.
It also saves work converting strings to mutable on deserialize,
although the performance difference seems negligible.
