Add 2.0 changes draft

docs/source/developer_zone/changes-2.0.rst

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+=================
+Mamba 2.0 Changes
+=================
+.. ...................... ..
+.. THIS IS STILL A DRAFT ..
+.. ...................... ..
+
+.. TODO high-level summary of new features:
+.. - OCI registries
+.. - Mirrors
+.. - Own implementation repodata.json
+.. - Fully feature implementation of MatchSpec
+
+
+Command Line Executables
+========================
+Mamba (executable)
+******************
+``mamba``, previously a Python executable mixing ``libmambapy``, ``conda``, and code to bridge both
+project is being replace by a fully C++ executable based ``libmamba`` solely.
+
+It now presents the same user interface and experience as ``micromamba``.
+
+.. warning::
+
+   The previous code being a highly complex project with few of its original developers still
+   active, it is hard to make an exhaustive list of all changes.
+
+   Please help us by reporting changes missing changes.
+
+Micromamba
+**********
+Micromamba recieves all new features and its CLI remains mostly unchanged.
+
+Breaking changes include:
+- ``micromamba shell init`` root prefix parameter ``--prefix`` (``-p``) was renamed
+  ``--root-prefix`` (``-r``).
+  Both options were supported in version ``1.5``.
+
+.. TODO is micromamba executable renamed mamba?
+
+
+Libraries
+=========
+Mamba (Python package)
+**********************
+In version 2.0, all tools are fully written in C++.
+All code previously available in Python through ``import mamba`` has been removed.
+
+Libmambapy (Python bindings)
+****************************
+The Python bindings to the C++ ``libamamba`` library remain available through ``import libmambapy``.
+They are now considered the first class citizen to using Mamba in Python.
+Changes inlcude:
+
+- The global ``Context``, previously available through ``Context()``, must now be accessed through
+  ``Context.instance()``.
+  What's more, it is required to be passed explicitly in a few more functions.
+  Future version of ``libmambapy`` will continue in this direction until there are no global context.In version 2, ``Context()`` will throw an exception to avoid hard to catch errors.
+- ``ChannelContext`` is no longer an implicit global variable.
+  It must be constructed with one of ``ChannelContext.make_simple`` or
+  ``ChannelContext.make_conda_compatible`` (with ``Context.instance`` as argument in most cases)
+  and passed explicitly to a few functions.
+- ``Channel`` has been redesigned an moved to a new ``libmambapy.specs``.
+  A featureful ``libmambapy.specs.CondaURL`` is used to describe channel URLs.
+  This module also includes ``ChannelSpec`` used to match channels.
+- ``MatchSpec`` has been redesigned and moved to ``libmambapy.specs``.
+  The module also includes a platform enumeration, an implementation of ordered ``Version``, and a
+  ``VersionSpec`` to match versions.
+
+.. TODO include final decision for Channels as URLs.
+
+Libmamba (C++)
+**************
+The C++ library ``libmamba`` has received significant changes.
+Due to the low usage of the C++ interface, all changes are not listed here.
+The main changes are:
+- Refactoring and testing of a large number of utilities into a ``util::`` namespace,
+- Creation of the ``specs::`` with:
+    - Implementations of ``Version`` and ``VersionSpec`` for matching versions,
+    - A refactoring of a purely funcitonal ``Channel`` class,
+    - Implementaiton of a ``ChannelSpec`` to match ``Channels``,
+    - A refactored implementation of ``MatchSpec`` using the components above.
+- A cleanup of ``ChannelContext`` for be a light proxy and parameter holder wrapping the
+  ``specs::Channel``.
+- A new ``repodata.json`` parser using `simdjson <https://simdjson.org/>`_.
+- Improved downloaders.
+
+.. TODO OCI registry
+.. TODO Mirrors