move generic graph-colouring code into util

It is needed by cmm/StackColor, and hence is needed even when there is no
native code generator.

diff --git a/compiler/nativeGen/GraphBase.hs b/compiler/utils/GraphBase.hs
similarity index 100%
rename from compiler/nativeGen/GraphBase.hs
rename to compiler/utils/GraphBase.hs

diff --git a/compiler/nativeGen/GraphColor.hs b/compiler/utils/GraphColor.hs
similarity index 100%
rename from compiler/nativeGen/GraphColor.hs
rename to compiler/utils/GraphColor.hs

diff --git a/compiler/nativeGen/GraphOps.hs b/compiler/utils/GraphOps.hs
similarity index 100%
rename from compiler/nativeGen/GraphOps.hs
rename to compiler/utils/GraphOps.hs

diff --git a/compiler/nativeGen/GraphPpr.hs b/compiler/utils/GraphPpr.hs
similarity index 100%
rename from compiler/nativeGen/GraphPpr.hs
rename to compiler/utils/GraphPpr.hs