The tightest upper bound one can specify on the number of fields in a
struct is `sizeof(type) * CHAR_BIT`. So this was previously used when
performing a binary search for the field count. This upper bound is
extremely loose when considering a typical large struct, which is more
likely to contain a relatively small number of relatively large fields
rather than the other way around. The binary search range being multiple
orders of magnitude larger than necessary wouldn't have been a
significant issue if each test was cheap, but they're not. Testing a
field count of N costs O(N) memory and time. As a result, the initial
few steps of the binary search may be prohibitively expensive.

The primary optimization introduced by these changes is to use unbounded
binary search, a.k.a. exponential search, instead of the typically
loosely bounded binary search. This produces a tight upper bound (within
2x) on the field count to then perform the binary search with.

As an upside of this change, the compiler-specific limit placed on the
upper bound on the field count to stay within compiler limits could be
removed.

In the last CI run, 15 tasks failed with a compiler is out of heap space error.
With the jobs running in parallel, it's hard to determine which tasks failed due
to their own excessive memory usage and which were well-behaved, but a victim of
running when another task consumed all the available memory.

This could happen for a type with a constructor accepting a parameter
pack.

This also prevents unbounded growth in case something goes wrong with
the logic and something should have already stopped (or never started).

Makes the evaluation of the field count of huge arrays not result in
excessive compiler resource utilization.

Makes the evaluation of the field count of huge arrays not result in
excessive compiler resource utilization.

Verified ok targeting both x86 and x86-64 for:
- clang 3.7.1
- clang 19.1.0
- gcc 8.2
- gcc 14.2
- msvc 19.20
- msvc 19.40

This reverts commit a6d0c5f.

Conflicts:
- include/boost/pfr/detail/fields_count.hpp

…econditions"

This reverts commit 096445f.

Conflicts:
- include/boost/pfr/detail/fields_count.hpp