Change: Improve performance of finding free pool slots. (#12055)

Add a bitmap of used pool slots which allows finding a free pool slot without having to check if each index is already used or not.

Loosely based on a JGRPP patch.

Codechange: automatic adding of _t to (u)int types, and WChar to char32_t

for i in `find src -type f|grep -v 3rdparty/fmt|grep -v 3rdparty/catch2|grep -v 3rdparty/opengl|grep -v stdafx.h`; do sed 's/uint16& /uint16 \&/g;s/int8\([ >*),;[]\)/int8_t\1/g;s/int16\([ >*),;[]\)/int16_t\1/g;s/int32\([ >*),;[]\)/int32_t\1/g;s/int64\([ >*),;[]\)/int64_t\1/g;s/ uint32(/ uint32_t(/g;s/_uint8_t/_uint8/;s/Uint8_t/Uint8/;s/ft_int64_t/ft_int64/g;s/uint64$/uint64_t/;s/WChar/char32_t/g;s/char32_t char32_t/char32_t WChar/' -i $i; done

Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

Codechange: Remove std::function from Pool iteration wrapper

Add a separate template wrapper for filtered iteration

(svn r18045) -Fix: GCC 4.5@HEAD not compiling OpenTTD anymore because of a "non-placement deallocation function [is] selected for placement delete", or in other words delete(void *, size_t) is 'magic'.
We implemented these delete(void *, size_t) operator functions because MSVC warned that "no matching operator delete found; memory will not be freed if initialization throws an exception" for new(size_t, size_t).
This disables MSVC warning about this because we do not use exceptions in the (constructors that use the) overridden allocation functions, as such they will never be called; delete(void *) remains necessary though.