mv difftree/internal/radixmerkle to difftre/internal/merkletrie (#138)

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diff --git a/difftree/internal/merkletrie/doc.go b/difftree/internal/merkletrie/doc.go
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+package merkletrie
+
+/*
+Package merkletrie gives support for n-ary trees that are at the same
+time Merkle trees and Radix trees, and provides an efficient tree
+comparison algorithm for them.
+
+Git trees are Radix n-ary trees in virtue of the names of their
+tree entries.  At the same time, git trees are Merkle trees thanks to
+their hashes.
+
+When comparing git trees, the simple approach of alphabetically sorting
+their elements and comparing the resulting lists is not enough as it
+depends linearly on the number of files in the trees: When a directory
+has lots of files but none of them has been modified, this approach is
+very expensive.  We can do better by prunning whole directories that
+have not change, by just by looking at their hashes.  This package
+provides the tools to do exactly that.
+
+This package defines Radix-Merkle trees as nodes that should have:
+- a hash: the Merkle part of the Radix-Merkle tree
+- a key: the Radix part of the Radix-Merkle tree
+
+The Merkle hash condition is not enforced by this package though.  This
+means that node hashes doesn't have to take into account the hashes of
+their children,  which is good for testing purposes.
+
+Nodes in the Radix-Merkle tree are abstracted by the Noder interface.
+The intended use is that git.Tree implements this interface.
+*/