Tests fail to compile for 0.3.2.1

[sternenseemann]

Seems like an instance declaration was left over that has been moved into the library:

Building test suite 'test' for mfsolve-0.3.2.1..
[1 of 1] Compiling Main             ( tests/test.hs, dist/build/test/test-tmp/Main.o )

tests/test.hs:52:10: error:
    Duplicate instance declarations:
      instance (Floating n, Eq n, Ord n, Ord v) => Eq (Expr v n)
        -- Defined at tests/test.hs:52:10
      instance (Eq n, Eq v) => Eq (Expr v n) -- Defined in ‘Math.MFSolve’
   |
52 | instance (Floating n, Eq n, Ord n, Ord v) => Eq (Expr v n) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^