Add `Version.precedence_key`.

This will be used in `sort(..., key=lambda v: v.precedence_key)`.
Remove previous comparison/ordering implementation.

The current implementation relies on a 4-tuple:
- major, minor, patch (as integers) - natural order matches precedence
  rules
- a tuple of identifiers for the prerelease component.

The identifiers for the prerelease components are based on:
- A `NumericIdentifier` class, that compares number using their natural
  order, and always compares lower than non-numeric identifiers
- A `AlphaIdentifier` class for non-numeric identifiers; it compares
  versions using ASCII ordering.
- A `MaxIdentifier` class, that compares higher to any other identifier;
  used to ensure that a non-prerelease version is greater than any of
  its prereleases.

1 files changed, 2 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 31ee8aa..9fbcae6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,8 @@ ChangeLog
     * Add ``Version.truncate()`` to build a truncated copy of a ``Version``
     * Add ``NpmSpec(...)``, following strict NPM matching rules (https://docs.npmjs.com/misc/semver)
     * Add ``Spec.parse('xxx', syntax='<syntax>')`` for simpler multi-syntax support
+    * Add ``Version().precedence_key``, for use in ``sort(versions, key=lambda v: v.precedence_key)`` calls.
+      The contents of this attribute is an implementation detail.
 
 *Bugfix:*