[plt-scheme] new minor release of PLT Redex

I've just sent in v2.6 of redex.plt. In the last few releases there
have been bunch of improvements and this will probably be the last
minor version before some backwards-incompatible changes. These are the
highlights from the last few versions:

  - added a bunch of new patterns for things are commonly written as
    side-conditions: support for a kind of "namespace" (prefixed
    variables, technically), patterns that ensure named sub-expressions
    must be different from each other, and ways to make sure different
    sequences have the same (or different) length.

  - significantly improved the memory consumption (added
    programmer-support for the pattern matching caches and better
    defaults) and also found a 30% speedup (altho the 30% speedup was
    back in 2.1)

  - added metafunction support, using the same notation as the
    reductions

  - added documentation that explains how to avoid performance bugs,
    and a note on a debugging technique.

  - added schemeunit support (two 'check's)

Robby