[plt-scheme] v300 on OpenBSD

The v300 OpenBSD problem turned out to be a bad equal-hash function for
inexact numbers in the case of +inf.0, -inf.0, or +nan.0.

The problem showed up when compiling parser-tools because the lexer
generator (run at compile time) uses an equal-based hash table with
keys that contain +inf.0. I don't know why the problem showed up only
with OpenBSD (and closely related systems).

This is fixed in the code in SVN. The current "src/mzscheme/src/hash.c"
can be used as a replacement for the "hash.c" that is distributed for
v301.

Matthew