adjust plai test suite tests to check the ports that things now go to
`equal?' hashing: shortcut and some caching for structure transparency
improve complexity of `hash-iterate-{key,value}'

For immutable hashes, `hash-iterate-{key,value}' used to take O(N) time on
the first call for a particular table and O(1) thereafter. Now it takes
O(1) time for all calls, the constant is only slightly larger for
the non-first calls, and there's no extra allocation.
change `eq?' hashing to avoid dropping the low bit for fixnums
Convert srfi/61/cond to use modern module style.

This ensures that `=>` and `else` have the same bindings as the ones in
`racket/private/cond`; otherwise it would not be possible to use SRFI 61
`cond` from a `#lang racket/base` program/module.

Tested under both `#lang racket` and `#lang mzscheme`.
fix bug in hashing shortcut for transparency
Remove duplicate non-terminals in `define-extended-language`

Fixes a problem with `define-union-language`
equality: struct transparency and inspector shortcuts and caching

Same as for hashing.
fix missing initialization of cached inspector reference

The 3m transformation fills it in, but it's obviously
better to be explicit and avoid the bug for CGC.
Some tweaks for Guide section 4
Improve Guide section 5
Guide chapter 6 tweaks
Use the `not-on-the-web' style *around* the installation comment.

Otherwise making it have a "display: none" style leaves a visible empty
div.
add macports default path as a place to look for aspell
Add contracts to net/dns functions.
Fix net/dns contracts