Prohibit using char without explicit sign modifier

Related to #79

.gitmodules

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+[submodule "test/pycparser"]
+	path = test/pycparser
+	url = https://github.com/eliben/pycparser.git

requirements.txt

@@ -1,3 +1,4 @@
 PyYAML
 nose
 rednose
+pycparser

test/pycparser

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+Subproject commit e1a1d737be66308b633215fa26ac5ed30e890103

test/test_char.py

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+
+"""
+Checks that the implementation does not make use of the `char` type.
+This is ambiguous; compilers can freely choose `signed` or `unsigned` char.
+"""
+
+import pqclean
+import pycparser
+import os
+
+
+def test_char():
+    for scheme in pqclean.Scheme.all_schemes():
+        for implementation in scheme.implementations:
+            yield check_char, implementation
+
+
+def walk_tree(ast):
+    if type(ast) is pycparser.c_ast.IdentifierType:
+        if ast.names == ['char']:
+            yield ast
+
+    for (_, child) in ast.children():
+        yield from walk_tree(child)  # recursively yield prohibited nodes
+
+
+def check_char(implementation):
+    errors = []
+    for fname in os.listdir(implementation.path()):
+        if not fname.endswith(".c"):
+            continue
+        tdir, _ = os.path.split(os.path.realpath(__file__))
+        ast = pycparser.parse_file(
+            os.path.join(implementation.path(), fname),
+            use_cpp=True,
+            cpp_args=[
+                '-nostdinc',  # pycparser cannot deal with e.g. __attribute__
+                '-I{}'.format(os.path.join(tdir, "../common")),
+                # necessary to mock e.g. <stdint.h>
+                '-I{}'.format(
+                    os.path.join(tdir, 'pycparser/utils/fake_libc_include')),
+            ]
+        )
+        for node in walk_tree(ast):
+            # flatten nodes to a string to easily enforce uniqueness
+            err = "\n at {c.file}:{c.line}:{c.column}".format(c=node.coord)
+            if err not in errors:
+                errors.append(err)
+    if errors:
+        raise AssertionError(
+            "Prohibited use of char without explicit signed/unsigned" +
+            "".join(errors)
+        )
+
+
+if __name__ == '__main__':
+    try:
+        import nose2
+        nose2.main()
+    except ImportError:
+        import nose
+        nose.runmodule()