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Binary reader and writer: remove checks that invariants make dead (#6001)

* Binary reader and writer: remove checks that invariants make dead

Branch coverage of the test suite and of 24k mutated inputs showed
these conditions as never taken; each is implied by a check made
earlier on the same path:

- check_bson_document_size: `document_size < 0` (the size was rejected
  below 5 before it was stored)
- get_ubjson_ndarray_size: `second != 'N'` ('N' is rejected as a type
  marker before `second` can be set)
- bjd_type_name() == nullptr for an ndarray dtype: every marker
  is_valid_optimized_type() accepts for BJData has a name (now an assert)
- the early is_bjd_excluded_optimized_type() check in
  get_ubjson_size_type(): is_valid_optimized_type() rejects the same
  eight markers with the same error
- `input_format == bjdata` next to `is_ndarray`, bit 8 of the type, or a
  'B'/'u'/'m'/'M' marker: these only arise for BJData
- `first != npos` next to bit 8: the bit is only set after a count
- MessagePack writer: the `<= max` halves of the negative int8/16/32
  ladder (the ladder is in the `else` of `n >= 0`)

No change in behaviour: a build that aborts on each removed side passes
the test suite and the fuzz corpora, and a 1.6M-input differential run
of the reader against the original headers found no difference.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Remove is_bjd_excluded_optimized_type(), which has no caller left; fix two comments

is_valid_optimized_type() rejects the markers BJData excludes, so the reader
no longer needs its own list; the test now checks is_valid_optimized_type()
of a BJData reader against the writer's list. The BSON size comment now says
that open_bson_document() rejects a size below 5.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Make the BJData reader in the lookup test const (misc-const-correctness)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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Signed-off-by: Niels Lohmann <mail@nlohmann.me>

24386 of 73638 branches covered (33.12%)

17 of 17 new or added lines in 2 files covered. (100.0%)

16647 of 16695 relevant lines covered (99.71%)

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