Project structure: extract examples (quicksort, foreign heap/list, ...) to internal library to allow easy run/test/benchmarking

[tbagrel1]

Using an internal library, in addition to the main library, seems to be supported by both Cabal and stack now, despite a warning from stack saying the opposite.

With this change, the examples test suite becomes the examples internal library, and I introduced test-examples to act as the test suite for shared examples. This allows us, for example, to also cherry pick some examples we want to benchmark, and share the code between the test suite and benchmark suite (which wasn't possible before).

Other changes:

• Renamed the default benchmark suite from mutable-data to bench, for consistency.
• Moved the genericTraverse tests from examples to the main test suite

This PR follows and builds upon #479

[aspiwack]

Up to small point below, merge when you're ready.

[aspiwack]

Suggested change

	test-suite test-examples
	test-suite examples

Should work. Because different units are namespaced between their types, so it'll be referred to as something like test:examples (instead of test:test-examples in your case).

[tbagrel1]

It doesnt seem to work:

library examples
    import: build-opts
    hs-source-dirs: examples
    
[...]

test-suite examples
    import: build-opts
    import: rts-opts-multithread
    type:             exitcode-stdio-1.0
    main-is:          Main.hs
    hs-source-dirs:   test/examples

gives

Configuring linear-base-0.4.0...
There exist several components with the same name: 'examples'. This is a bug in your .cabal file.
CallStack (from HasCallStack):
  error, called at libraries/Cabal/Cabal-syntax/src/Distribution/PackageDescription/Utils.hs:18:15 in Cabal-syntax-3.10.3.0:Distribution.PackageDescription.Utils
Progress 1/2

Error: [S-7282]
       Stack failed to execute the build plan.
       
       While executing the build plan, Stack encountered the error:
       
       [S-7011]
       While building package linear-base-0.4.0 (scroll up to its section to see the error)
       using:
[...]

[aspiwack]

My bad then.