Add preprocessor and unit_test modules
This change adds two Pigweed modules: pw_preprocessor and pw_unit_test.
The preprocessor module contains header files providing helpful macros
for the C preprocessor. The unit test module contains a starter
implementation of a unit testing framework for Pigweed.
Change-Id: I46e1a4cae1fd8ce36d7840a2e92f8013fb489cde
diff --git a/pw_preprocessor/docs.rst b/pw_preprocessor/docs.rst
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+.. _chapter-preprocessor:
+
+.. default-domain:: cpp
+
+.. highlight:: sh
+
+------------
+Preprocessor
+------------
+The preprocessor module provides various helpful preprocessor macros.
+
+Compatibility
+=============
+C and C++
+
+Dependencies
+============
+This module has no dependencies.
+
+Headers
+=======
+The preprocessor module provides several headers.
+
+pw_preprocessor/boolean.h
+-------------------------
+Defines macros for boolean logic on literal 1s and 0s. This is useful for
+situations when a literal is needed to build the name of a function or macro.
+
+pw_preprocessor/compiler.h
+--------------------------
+Macros for compiler-specific features, such as attributes or builtins.
+
+pw_preprocessor/concat.h
+------------------------
+Defines the ``PW_CONCAT(...)`` macro, which expands its arguments if they are
+macros and token pastes the results. This can be used for building names of
+classes, variables, macros, etc.
+
+pw_preprocessor/macro_arg_count.h
+---------------------------------
+Defines the ``PW_ARG_COUNT(...)`` macro, which counts the number of arguments it
+was passed. It can be invoked directly or with ``__VA_ARGS__`` in another macro.
+``PW_ARG_COUNT(...)`` evaluates to a literal of the number of arguments which
+can be used directly or concatenated to build other names. Unlike many common
+implementations, this macro correctly evaluates to ``0`` when it is invoked
+without arguments.
+
+This header also defines ``PW_HAS_ARGS(...)`` and ``PW_HAS_NO_ARGS(...)``,
+which evaluate to ``1`` or ``0`` depending on whether they are invoked with
+arguments.
+
+pw_preprocessor/util.h
+----------------------
+General purpose, useful macros.
+
+* ``PW_ARRAY_SIZE(array)`` -- calculates the size of a C array
+* ``PW_UNUSED(value)`` -- silences "unused variable" compiler warnings
+* ``PW_STRINGIFY(...)`` -- expands its arguments as macros and converts them to
+ a string literal
+* ``PW_EXTERN_C`` -- declares a name to be ``extern "C"`` in C++; expands to
+ nothing in C
+* ``PW_EXTERN_C_START`` / ``PW_EXTERN_C_END`` -- declares an ``extern "C" { }``
+ block in C++; expands to nothing in C