Draft: Finalize hi-haddock

This MR is essentially https://gitlab.haskell.org/ghc/haddock/-/tree/hi-haddock rebased onto https://gitlab.haskell.org/ghc/haddock/-/tree/ghc-head, with small fixes, some formatting/comments, and test fixes.

I've tested performance by generating documentation on the Agda codebase. With these patches, and those in the corresponding GHC patch, the -s statistics look like this:

  37,486,323,080 bytes allocated in the heap
  10,910,917,392 bytes copied during GC
     409,821,536 bytes maximum residency (31 sample(s))
       7,524,000 bytes maximum slop
            1145 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      8945 colls,     0 par    5.827s   5.964s     0.0007s    0.0378s
  Gen  1        31 colls,     0 par    3.890s   4.029s     0.1300s    0.4070s

  TASKS: 5 (1 bound, 4 peak workers (4 total), using -N1)

  SPARKS: 2 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 2 fizzled)

  INIT    time    0.004s  (  0.004s elapsed)
  MUT     time    9.045s  ( 10.824s elapsed)
  GC      time    9.717s  (  9.993s elapsed)
  EXIT    time    0.012s  (  0.011s elapsed)
  Total   time   18.779s  ( 20.832s elapsed)

  Alloc rate    4,144,346,991 bytes per MUT second

  Productivity  48.2% of total user, 52.0% of total elapsed

And the baseline (default haddock 9.6.1):

  40,312,405,088 bytes allocated in the heap
  12,933,710,968 bytes copied during GC
   1,034,675,264 bytes maximum residency (19 sample(s))
       7,060,416 bytes maximum slop
            2806 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      9620 colls,     0 par    5.840s   5.916s     0.0006s    0.0166s
  Gen  1        19 colls,     0 par    4.002s   4.236s     0.2230s    0.7593s

  TASKS: 5 (1 bound, 4 peak workers (4 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    9.700s  ( 10.783s elapsed)
  GC      time    9.842s  ( 10.153s elapsed)
  EXIT    time    0.013s  (  0.000s elapsed)
  Total   time   19.557s  ( 20.937s elapsed)

  Alloc rate    4,155,729,971 bytes per MUT second

  Productivity  49.6% of total user, 51.5% of total elapsed

Max residency is halved. This is even more of an improvement over the current ghc-head branch, which produces these statistics:

  37,312,388,296 bytes allocated in the heap
  11,240,716,688 bytes copied during GC
     555,377,456 bytes maximum residency (24 sample(s))
       6,524,112 bytes maximum slop
            1574 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      8914 colls,     0 par    6.011s   6.106s     0.0007s    0.0193s
  Gen  1        24 colls,     0 par    3.929s   4.148s     0.1728s    0.5674s

  TASKS: 5 (1 bound, 4 peak workers (4 total), using -N1)

  SPARKS: 2 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 2 fizzled)

  INIT    time    0.005s  (  0.005s elapsed)
  MUT     time    9.701s  ( 11.074s elapsed)
  GC      time    9.940s  ( 10.255s elapsed)
  EXIT    time    0.019s  (  0.012s elapsed)
  Total   time   19.665s  ( 21.345s elapsed)

  Alloc rate    3,846,058,284 bytes per MUT second

  Productivity  49.3% of total user, 51.9% of total elapsed