Make benchmarks compile with modern criterion

[RyanGlScott]

Currently, the benchmarks fail to build:

Preprocessing library xmlgen-0.6.2.1...
[1 of 1] Compiling Text.XML.Generator ( src/Text/XML/Generator.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/Text/XML/Generator.o )
In-place registering xmlgen-0.6.2.1...
Preprocessing benchmark 'xmlgen-bench' for xmlgen-0.6.2.1...
[1 of 1] Compiling Main             ( test/GeneratorBenchmarks.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/xmlgen-bench/xmlgen-bench-tmp/Main.o )

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:10:25:
    Couldn't match type ‘[Char]’ with ‘T.Text’
    Expected type: Name
      Actual type: [Char]
    In the first argument of ‘xelem’, namely ‘"root"’
    In the expression: xelem "root"
    In the expression:
      xelem "root"
      $ xelems
        $ map
            (\ s -> xelem "foo" (xattr "key" s, xtext s))
            (map (\ i -> T.pack (show i)) [1 .. numberOfElems])

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:10:60:
    Couldn't match type ‘[Char]’ with ‘T.Text’
    Expected type: Name
      Actual type: [Char]
    In the first argument of ‘xelem’, namely ‘"foo"’
    In the expression: xelem "foo" (xattr "key" s, xtext s)
    In the first argument of ‘map’, namely
      ‘(\ s -> xelem "foo" (xattr "key" s, xtext s))’

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:10:73:
    Couldn't match type ‘[Char]’ with ‘T.Text’
    Expected type: Name
      Actual type: [Char]
    In the first argument of ‘xattr’, namely ‘"key"’
    In the expression: xattr "key" s
    In the second argument of ‘xelem’, namely
      ‘(xattr "key" s, xtext s)’
/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:14:25:
    Couldn't match type ‘[Char]’ with ‘T.Text’
    Expected type: Name
      Actual type: [Char]
    In the first argument of ‘xelem’, namely ‘"root"’
    In the expression: xelem "root"
    In the expression:
      xelem "root"
      $ xattrs
        $ map
            (\ s -> xattr ("key-" ++ s) (T.pack s))
            (map (\ i -> show i) [1 .. numberOfElems])

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:14:61:
    Couldn't match type ‘[Char]’ with ‘T.Text’
    Expected type: Name
      Actual type: [Char]
    In the first argument of ‘xattr’, namely ‘("key-" ++ s)’
    In the expression: xattr ("key-" ++ s) (T.pack s)
    In the first argument of ‘map’, namely
      ‘(\ s -> xattr ("key-" ++ s) (T.pack s))’

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:21:74:
    Couldn't match expected type ‘Benchmarkable’
                with actual type ‘IO ()’
    In the second argument of ‘bench’, namely ‘(benchElems i)’
    In the expression: bench (show i ++ " elems") (benchElems i)

/nfs/home/crest-team/jenkins_workspaces/cutter/workspace/benchmark_stackage/label/CREST_cutter/.bench-build/lts-5.16/xmlgen-0.6.2.1/test/GeneratorBenchmarks.hs:22:74:
    Couldn't match expected type ‘Benchmarkable’
                with actual type ‘IO ()’
    In the second argument of ‘bench’, namely ‘(benchAttrs i)’
    In the expression: bench (show i ++ " attrs") (benchAttrs i)

I made some small tweaks to get things working again:

• Use whnfIO from criterion >= 1
• Account for the existence of Text in more places

See also iu-parfunc/sc-haskell#7

[RyanGlScott]

Closing stale PR.