Cleanup includes

[BenBE]

This updates all source files to mention exactly those includes required for compiling.

Tested with IWYU 0.12 + clang 9. Please report issues with other setups.

NB: IWYU sometimes has issues with finding certain declarations and reports compile issues in these cases. If plain clang compiles then this is an issue with IWYU. Such issues can usually be ignored.

FWIW: IWYU sees functions used inside defines as being used where the macro is used, not where the define is declared. This is a good reason to adopt #154 to avoid this issue.

As I'm sure there will be compile issues on some platforms with this as is I'm just drafting this for now.

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[BenBE]

The order of includes I established in this PR is:

1. "config.h" with a IWYU pragma to always keep it (required for conditional defines and certain system macros)
2. header for the file being compiled
3. all system headers in alphabetical order, sub directories at the end
4. all our headers in alphabetical order, sub directories at the end
5. all conditionally included headers, i.e. checked by #if defined(…)

AFTER the block of includes two blank lines should be present before anything else.

For header files item 2 is simply skipped.

Important note: For functions used as part of macro defines you must add an IWYU pragma to keep these headers. Otherwise IWYU will suggest to remove them as unused AND ask each and every user of a define to include those headers. This is BTW another good reason to avoid function-like defines in headers and replace them as inline functions.

[natoscott]

The order of includes I established in this PR is:
[...]

LGTM. The 'run_iwyu.sh' script looks like it uses bash-isms - perhaps it should use a /bin/bash shebang line instead of /bin/sh? (the run_valgrind script has the same issue, so I'm gonna ignore it for now - feel free to follow up though)

[...]
This is BTW another good reason to avoid function-like defines in headers and replace them as inline functions.

Hmm - that sounds more like a bug in IWYU than a good reason to do anything, TBH.