merge(3p/immer): Subtree merge at 'ad3e3556d' as 'third_party/immer'

Change-Id: I9636a41ad44b4218293833fd3e9456d9b07c731b

third_party/immer/doc/algorithms.rst

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+
+Algorithms
+==========
+
+This module provides overloads of standard algorithms that leverage
+the internal structure of the immutable containers to provide faster
+iteration. These are drop-in replacements of the respective STL
+algorithms that can be a few times faster when applied on immutable
+sequences.
+
+For further convenience they can be passed in just a container where
+the standard library algorithms require being passed in two iterators.
+
+.. note::
+
+   They are a similar idea to `structure-aware iterators`_
+   but implemented using higher order functions in order to support
+   structures of any depth.  The downside is that this sometimes
+   causes the compiler to generate bigger executable files.
+
+.. _structure-aware iterators: https://www.youtube.com/watch?v=T3oA3zAMH9M
+
+
+-----
+
+.. doxygengroup:: algorithm
+   :project: immer
+   :content-only: