repository: add ChunkerFactory and decouple archiver from chunker

Introduce restic.Chunker and ChunkerFactory interfaces with a
repository-backed implementation. The archiver obtains chunkers via
ChunkerFactory() from the repository.

internal/repository/chunker.go

@@ -0,0 +1,39 @@
+package repository
+
+import (
+	"github.com/restic/chunker"
+	"github.com/restic/restic/internal/restic"
+)
+
+type baseChunker struct {
+	bc  *chunker.BaseChunker
+	pol chunker.Pol
+}
+
+func (c *baseChunker) Reset() {
+	c.bc.Reset(c.pol)
+}
+
+func (c *baseChunker) NextSplitPoint(buf []byte) int {
+	return c.bc.NextSplitPoint(buf)
+}
+
+type chunkerFactory struct {
+	pol chunker.Pol
+}
+
+func newChunkerFactory(pol chunker.Pol) *chunkerFactory {
+	return &chunkerFactory{pol: pol}
+}
+
+func (f *chunkerFactory) NewChunker() restic.Chunker {
+	return &baseChunker{bc: chunker.NewBase(f.pol), pol: f.pol}
+}
+
+func (f *chunkerFactory) MaxChunkSize() int {
+	return chunker.MaxSize
+}
+
+func (r *Repository) ChunkerFactory() restic.ChunkerFactory {
+	return newChunkerFactory(r.Config().ChunkerPolynomial)
+}