Remove unreachable branches from the binary writer (#5583)

Coverage reported conditions in the binary writer that can never be
false, and marked the code behind them with LCOV_EXCL. Remove them
instead of excluding them:

- CBOR writes the length of a string, binary value, array, or object
  exactly like an unsigned integer, only with another major type. One
  function, write_cbor_head(), now writes both, so the integer tests
  cover every width and the four excluded 64-bit length branches are
  gone.
- A last `else if` whose condition holds for every remaining value
  (an unsigned value at most UINT64_MAX, a signed one in the range of
  int64_t) is now a plain `else`.
- Whether a signed integer fits into an int64 for UBJSON and BJData is
  decided by its type at compile time. Only an integer type wider than
  64 bits gets a range check and the high-precision fallback.
- The private get_impl(boolean_t*) was never called.

The UBJSON type prefix 'H' of an optimized container of unsigned
integers beyond the range of int64 was reachable although excluded; it
is tested now.

The output is unchanged.

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

include/nlohmann/json.hpp

@@ -2157,17 +2157,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     // value access //
     //////////////////
 
-    /// get a boolean (explicit)
-    boolean_t get_impl(boolean_t* /*unused*/) const
-    {
-        if (JSON_HEDLEY_LIKELY(is_boolean()))
-        {
-            return m_data.m_value.boolean;
-        }
-
-        JSON_THROW(type_error::create(302, detail::concat("type must be boolean, but is ", type_name()), this));
-    }
-
     /// get a pointer to the value (object)
     object_t* get_impl_ptr(object_t* /*unused*/) noexcept
     {