Change: include fmt.h C++ headers in stdafx.h

This to prevent compilation issues between runs with and without precompiled
headers. Also remove the headers from the rest of the code base as they are
not needed there anymore, although they do relatively little harm.

Remove: AutoFreeSmallVector.

The last use was for storing a list of memory blocks. As the way these lists are accessed is very
specific, it is easier to just write an explicit destructor instead of trying to exactly match the behaviour.

Codechange: Switch DropDownList to directly use std::vector, thus making AutoDeleteSmallVector obsolete.

DropDownListItem are strongly managed using std::unique_ptr to ensure leak-free handling. Appropriate use
of move-semantics make intent a lot clearer than parameter comments and allows the compiler to generate
copy-free code for most situations.

Codechange: Replaced SmallVector::Find() const with suitable alternatives

The use of std::none_of in network/core/host.cpp is driven by the non-const
comparison operator use by NetworkAddress. A future commit should address
the const_casts in that class to ensure const-correctness.

Codechange: [core] Implement SmallVector using std::vector

The public and protected interface to SmallVector are unchanged
SmallVector now requires that items be default constructible
This isn't an issue since some contained items were previously created
uninitialized.

Temporary default constructors are added to the following structs
- SmallPair
- SmallStackItem
- GRFPresence

Where vector<bool> is required, transition immediately to std::vector
to avoid returning proxy object references.

(svn r18640) -Fix (r18633): don't assert when accessing first invalid item, it's used quite often in the code
-Revert (r18637): it's not needed any more

(svn r13575) -Codechange: Move small vector to core since it fits better in there
-Codechange: convert smallvector from struct to class