Insert links to previous releases in (some) release notes files

Mention move and rename of old ordinalTheory in release notes.

Improve rel notes' example of Define's ability to handle consts as params.

Previous version may have given impression that re-used constant still had to have same type as parameter. Example now has

Define`x = (2,3)`;
Define`f x = x + 1`;

Update description of ordinal example in the release notes.

Two new bullet points in K8 release notes.

Minor formatting changes to Tarek's notes on new probability theories.

Entry for probability and real_sigmaTheory.

Entry for probability and real_sigmaTheory.

Fix printing of unnecessary guff on theory load with Unicode off.

Move n2l and l2n constants (and similar) into new numposrep theory.

A number of fixes follow on from this.

Also mention the new theories (numposrep and ASCIInumbers) in the release notes.

This is progress with #70.

Mention gcdsetTheory and ordinalTheory in release notes.

Change release notes stylesheet to make <code> font normal-size.

Credit Scott and link to github issue in release notes for #17.

Document fix of #73 in release notes.

Finish a sentence in the release notes with a full-stop.

Don't emit definition bindings for enumerated constants.

Fixes #78.

This commit also

* fixes uses of constructor definitions (the equations are still
present under the "number2<ty>_thm" name)
* provides a regression test
* a bug fix claim in the release notes.

Replace ugly hexadecimal HTML entity with Unicode universal quantifier.

Alter Define's handling of function argument variables. Closes #17.

Document new PROD_IMAGE and PROD_SET constants in release notes.

Also perform overloading to Unicode Pi and PI.

Fix footer of K-8 release notes.

Document ability to use a leading bar on the first case of a case-expr.

Make case-expressions in set comprehensions print with parentheses.

The way this is made to work is rather hackish: every possible case
expression in a comprehension gets parenthesised, even if it won't
cause a parse error. I don't expect there are many such in any case.

Describe change to case-syntax in release notes.