Add package level doc comments

container/doc.go

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+// Package ucontainer extends container and implements generic container types.
+package ucontainer

image/color/doc.go

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+// Package ucolor extends image/color.
+package ucolor

image/path/doc.go

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+// Package upath implements operations on slices of image.Points.
+package upath

io/doc.go

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+// Package uio extends io.
+package uio