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.
