[autofit] Split `afwrtsys.h`.

The header file `afwrtsys.h` has two distinct functions: to include the
required writing system headers and also to generate code for each writing
system.  At each current use site only one or the other is used, with
various macro trickery selecting one or the other.  Split this header into
`afws-decl.h` for the required writing system declarations and `afws-iter.h`
for iterating over the writing systems to generate code.

The motivation for this change is that the Visual C++ compiler treats the
standard include guard idiom like `#pragma once` 'if no non-comment code or
preprocessor directive comes before or after the standard form of the
idiom'.  It appears to check this after macro expansion, so if
`WRITING_SYSTEM` expands to empty the bottom of `afwrtsys.h` is empty and
looks like the standard include guard idiom which is treated like `#pragma
once`, so subsequent inclusion of `afwrtsys.h` is elided.

Fixes #1075.

* src/autofit/afglobal.c (af_writing_system_classes), src/autofit/aftypes.h
(AF_WritingSystem), src/autofit/rules.mk (AUTOF_DRV_H): Updated.

* src/autofit/afwrtsys.h: Split into...
* src/autofit/afws-decl.h, src/autofit/afws-iter.h: New files.

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