[racket-dev] literal strings and numbers are now interned

As of v5.2.0.4, Racket's reader and `datum->syntax' interns literal
strings, byte strings, regexps, characters, and numbers. (Also,
`equal?' now works in the obvious way for regexps.)

For example, `(eq? "hello" "hello")' will always return true, since the
two literal "hello"s are interned to the same string. Of course, it's
best not to write code that depends on this subtle fact, but the
reader's interning behavior is specified.

The change is intended to simplify cross-module optimization. The
compiler can now inline an inexact number bound to `pi', for example,
without potentially changing the semantics of a program that uses
something like `(eq? pi pi)'. Probably no good program would break with
cross-module inlining of numbers even without interning, but it seems
simplest to dispense with the question by always interning.

Along the same lines, I think it's unlikely that good programs are
affected by the `read' interning change, although I had to fix some
Racket tests that specifically check `eq?'-related behavior. Since
`datum->syntax' also interns literals, it's possible that the change
defeats some use of 3-D syntax that depends on the identity of a string
or number; again, though, that seems unlikely.

Finally, it's unlikely that interning alone will provide any
performance improvement. Fewer copies of strings and numbers may be
retained, but those savings probably just pay for the overhead of the
intern table.