add the two key graph modules from Martin Erwig's FGL

Martin Erwig's FGL (Functional Graph Library) provides an "inductive"
representation of graphs.  A general graph has labeled nodes and
labeled edges.  The key operation on a graph is to decompose it by
removing one node, together with the edges that connect the node to
the rest of the graph.  There is also an inverse composition
operation.

The decomposition and composition operations make this representation
of graphs exceptionally well suited to implement graph algorithms in
which the graph is continually changing, as alluded to in #21259.

This commit adds `GHC.Data.Graph.Inductive.Graph`, which defines the
interface, and `GHC.Data.Graph.Inductive.PatriciaTree`, which provides
an implementation.  Both modules are taken from `fgl-5.7.0.3` on
Hackage, with these changes:

  - Copyright and license text have been copied into the files
    themselves, not stored separately.

  - Some calls to `error` have been replaced with calls to `panic`.

  - Conditional-compilation support for older versions of GHC,
    `containers`, and `base` has been removed.

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