value: Drop distinction between long and short string values.

The time has passed when this is a super valuable distinction.  There is
basically no distinction in the language itself anymore, the efficiency
gains sound pretty marginal, and the differences lead to bugs, both
correctness- and memory leak-wise, that are hard to test for (because the
developer must make sure to test both short and long strings everywhere).
This commit changes the code so that both short and long strings are
represented the way that long strings were previously.