Move always.h from headers to cpp files

[xezon]

This change aims to further reduce compile times.

On my machine with SSD, this reduces compile time in Debug configuration by 1 second - from 42 seconds to 41 seconds.

Disadvantage of this change

Most headers will no longer compile on their own when included in a cpp file. Every cpp file now needs to have always.h included at the very top. Our clang-format already sorts it that way.

Advantage of this change

This principle causes less file includes and a bit faster compilation.

Also, it allows the project to use precompiled headers moving forward. For next steps, we can move always.h into a stdafx.h, and then bundle it with some other hot files such as xfer.h and asciistring.h, and include stdafx.h instead of always.h in all cpp files.

https://en.wikipedia.org/wiki/Precompiled_header

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.54%. Comparing base (b013231) to head (4c6df87).

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1121   +/-   ##
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  Coverage     2.54%    2.54%           
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  Files          949      949           
  Lines       110299   110299           
  Branches     18881    18881           
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  Hits          2802     2802           
  Misses      107094   107094           
  Partials       403      403           

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[OmniBlade]

Okay, so my concerns are 1. I think headers compiling on their own is a useful feature and they should include anything they require. 2. The precompiled header benefits only apply to Windows while we aspire to have the code base compile cross platform. In a similar project https://github.com/OmniBlade/LasMarionetas/tree/master/src/compat I've split what always.h encapsulates into multiple headers and just avoided the macros for platforms in favour of the compiler provided ones.

[xezon]

I don't feel strongly about this change. We can close it.

As for multi platform, Windows by far is the most important platform because this is what 99% of the actual players use, so any development improvements for Windows platform are worthwhile improvements.